diff --git a/src/domains/rules/NoMergeCommits.tests.ts b/src/domains/rules/NoMergeCommits.tests.ts
new file mode 100644
index 00000000..7f95378d
--- /dev/null
+++ b/src/domains/rules/NoMergeCommits.tests.ts
@@ -0,0 +1,71 @@
+import { describe, expect, it } from "vitest"
+import { fakeCommitFactory } from "#commits/Commit.fixtures.ts"
+import { fakeConfiguration } from "#configurations/Configuration.fixtures.ts"
+import { commitConcern } from "#rules/concerns/CommitConcern.ts"
+import type { Concerns } from "#rules/concerns/Concern.ts"
+import { noMergeCommits } from "#rules/NoMergeCommits.ts"
+import { ruleContext } from "#rules/Rule.ts"
+import { fakeCommitSha } from "#types/CommitSha.fixtures.ts"
+import type { CommitSha } from "#types/CommitSha.ts"
+
+const enabled = ruleContext("noMergeCommits")
+
+const fakeCommit = fakeCommitFactory(fakeConfiguration())
+
+describe.each`
+	parents                                                | subjectLine
+	${[fakeCommitSha(), fakeCommitSha()]}                  | ${"Merge branch 'main' into bugfix/dance-party-playlist"}
+	${[fakeCommitSha(), fakeCommitSha()]}                  | ${"Merge pull request #440 from renovate/battle-shell-7.x"}
+	${[fakeCommitSha(), fakeCommitSha(), fakeCommitSha()]} | ${"amend! Merge branch 'feature/new-coffee-machine' into feature/office-overhaul"}
+	${[fakeCommitSha(), fakeCommitSha(), fakeCommitSha()]} | ${"Keep my branch up to date"}
+`(
+	"when the commit is a merge commit with $parents.length parents and has a subject line of $subjectLine",
+	(props: { parents: Array<CommitSha>; subjectLine: string }) => {
+		const commit = fakeCommit({ message: props.subjectLine, parents: props.parents })
+
+		describe("and the rule is enabled", () => {
+			const actualConcerns = noMergeCommits([commit], enabled.options)
+
+			it("raises a concern about the entire commit", () => {
+				expect(actualConcerns).toEqual<Concerns>([commitConcern(enabled, commit.sha)])
+			})
+		})
+
+		describe("and the rule is disabled", () => {
+			const actualConcerns = noMergeCommits([commit], null)
+
+			it("does not raise any concerns", () => {
+				expect(actualConcerns).toEqual<Concerns>([])
+			})
+		})
+	},
+)
+
+describe.each`
+	parents              | subjectLine
+	${[]}                | ${"init"}
+	${[]}                | ${"Establish the repository"}
+	${[fakeCommitSha()]} | ${"just an ordinary commit"}
+	${[fakeCommitSha()]} | ${"Release the robot butler"}
+`(
+	"when the commit is not a merge commit and has a subject line of $subjectLine",
+	(props: { parents: Array<CommitSha>; subjectLine: string }) => {
+		const commit = fakeCommit({ message: props.subjectLine, parents: props.parents })
+
+		describe("and the rule is enabled", () => {
+			const actualConcerns = noMergeCommits([commit], enabled.options)
+
+			it("does not raise any concerns", () => {
+				expect(actualConcerns).toEqual<Concerns>([])
+			})
+		})
+
+		describe("and the rule is disabled", () => {
+			const actualConcerns = noMergeCommits([commit], null)
+
+			it("does not raise any concerns", () => {
+				expect(actualConcerns).toEqual<Concerns>([])
+			})
+		})
+	},
+)
diff --git a/src/domains/rules/NoMergeCommits.ts b/src/domains/rules/NoMergeCommits.ts
index 7e31369c..0ed815f1 100644
--- a/src/domains/rules/NoMergeCommits.ts
+++ b/src/domains/rules/NoMergeCommits.ts
@@ -1,9 +1,16 @@
 import type { Commit, Commits } from "#commits/Commit.ts"
+import { commitConcern } from "#rules/concerns/CommitConcern.ts"
 import type { Concern, Concerns } from "#rules/concerns/Concern.ts"
 import { type RuleContext, ruleContext } from "#rules/Rule.ts"
 import type { EmptyObject } from "#types/EmptyObject.ts"
 import { notNullish } from "#utilities/Arrays.ts"
 
+/**
+ * Verifies that the commit has at most one parent commit, thus disallowing merge commits.
+ *
+ * Avoiding merge commits makes the commit history linear and preserves the ability to rebase interactively.
+ * This helps to preserve the readability of the commit history and makes it easier to revert changes later.
+ */
 export function noMergeCommits(commits: Commits, options: EmptyObject | null): Concerns {
 	if (options === null) {
 		return []
@@ -13,6 +20,10 @@ export function noMergeCommits(commits: Commits, options: EmptyObject | null): C
 	return commits.map((commit) => verifyCommit(commit, rule)).filter(notNullish)
 }
 
-function verifyCommit(_commit: Commit, _rule: RuleContext): Concern | null {
-	throw new Error("The `noMergeCommits` rule has not been implemented yet") // TODO: To be implemented.
+function verifyCommit(commit: Commit, rule: RuleContext): Concern | null {
+	if (commit.isMergeCommit) {
+		return commitConcern(rule, commit.sha)
+	}
+
+	return null
 }
diff --git a/src/domains/rules/NoSquashMarkers.ts b/src/domains/rules/NoSquashMarkers.ts
index 242b5307..213b95f4 100644
--- a/src/domains/rules/NoSquashMarkers.ts
+++ b/src/domains/rules/NoSquashMarkers.ts
@@ -7,7 +7,7 @@ import type { EmptyObject } from "#types/EmptyObject.ts"
 import { notNullish } from "#utilities/Arrays.ts"
 
 /**
- * Verifies that the subject line does not contain squash markers.
+ * Verifies that the subject line does not contain any squash marker.
  *
  * Combining squash commits with their ancestors makes the commit history cleaner and easier to read,
  * as it omits unnecessary diffs and increases the cohesion of commits. It also makes it easier to revert changes later.
diff --git a/src/domains/rules/UseConciseSubjectLines.tests.ts b/src/domains/rules/UseConciseSubjectLines.tests.ts
index 205aa173..5256e16a 100644
--- a/src/domains/rules/UseConciseSubjectLines.tests.ts
+++ b/src/domains/rules/UseConciseSubjectLines.tests.ts
@@ -33,8 +33,9 @@ describe.each`
 		const commit = fakeCommit({ message: props.subjectLine })
 
 		describe("and the rule is enabled with a maximum length of 20 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled20.options)
+
 			it("raises a concern about the characters that exceed the limit", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled20.options)
 				expect(actualConcerns).toEqual<Concerns>([
 					subjectLineConcern(enabled20, commit.sha, { range: props.expectedRange20 }),
 				])
@@ -42,8 +43,9 @@ describe.each`
 		})
 
 		describe("and the rule is enabled with a maximum length of 50 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled50.options)
+
 			it("raises a concern about the characters that exceed the limit", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled50.options)
 				expect(actualConcerns).toEqual<Concerns>([
 					subjectLineConcern(enabled50, commit.sha, { range: props.expectedRange50 }),
 				])
@@ -51,8 +53,9 @@ describe.each`
 		})
 
 		describe("and the rule is enabled with a maximum length of 72 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled72.options)
+
 			it("raises a concern about the characters that exceed the limit", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled72.options)
 				expect(actualConcerns).toEqual<Concerns>([
 					subjectLineConcern(enabled72, commit.sha, { range: props.expectedRange72 }),
 				])
@@ -60,8 +63,9 @@ describe.each`
 		})
 
 		describe("and the rule is disabled", () => {
+			const actualConcerns = useConciseSubjectLines([commit], null)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], null)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
@@ -84,8 +88,9 @@ describe.each`
 		const commit = fakeCommit({ message: props.subjectLine })
 
 		describe("and the rule is enabled with a maximum length of 20 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled20.options)
+
 			it("raises a concern about the characters that exceed the limit", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled20.options)
 				expect(actualConcerns).toEqual<Concerns>([
 					subjectLineConcern(enabled20, commit.sha, { range: props.expectedRange20 }),
 				])
@@ -93,8 +98,9 @@ describe.each`
 		})
 
 		describe("and the rule is enabled with a maximum length of 50 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled50.options)
+
 			it("raises a concern about the characters that exceed the limit", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled50.options)
 				expect(actualConcerns).toEqual<Concerns>([
 					subjectLineConcern(enabled50, commit.sha, { range: props.expectedRange50 }),
 				])
@@ -102,15 +108,17 @@ describe.each`
 		})
 
 		describe("and the rule is enabled with a maximum length of 72 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled72.options)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled72.options)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
 
 		describe("and the rule is disabled", () => {
+			const actualConcerns = useConciseSubjectLines([commit], null)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], null)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
@@ -135,8 +143,9 @@ describe.each`
 		const commit = fakeCommit({ message: props.subjectLine })
 
 		describe("and the rule is enabled with a maximum length of 20 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled20.options)
+
 			it("raises a concern about the characters that exceed the limit", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled20.options)
 				expect(actualConcerns).toEqual<Concerns>([
 					subjectLineConcern(enabled20, commit.sha, { range: props.expectedRange20 }),
 				])
@@ -144,22 +153,25 @@ describe.each`
 		})
 
 		describe("and the rule is enabled with a maximum length of 50 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled50.options)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled50.options)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
 
 		describe("and the rule is enabled with a maximum length of 72 characters", () => {
+			const actualConcerns = useConciseSubjectLines([commit], enabled72.options)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], enabled72.options)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
 
 		describe("and the rule is disabled", () => {
+			const actualConcerns = useConciseSubjectLines([commit], null)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], null)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
@@ -185,16 +197,18 @@ describe.each`
 		`(
 			"and the rule is enabled with a maximum length of $options.maxLength characters",
 			(context: RuleContext<"useConciseSubjectLines">) => {
+				const actualConcerns = useConciseSubjectLines([commit], context.options)
+
 				it("does not raise any concerns", () => {
-					const actualConcerns = useConciseSubjectLines([commit], context.options)
 					expect(actualConcerns).toEqual<Concerns>([])
 				})
 			},
 		)
 
 		describe("and the rule is disabled", () => {
+			const actualConcerns = useConciseSubjectLines([commit], null)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], null)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
@@ -218,16 +232,18 @@ describe.each`
 		`(
 			"and the rule is enabled with a maximum length of $options.maxLength characters",
 			(context: RuleContext<"useConciseSubjectLines">) => {
+				const actualConcerns = useConciseSubjectLines([commit], context.options)
+
 				it("does not raise any concerns", () => {
-					const actualConcerns = useConciseSubjectLines([commit], context.options)
 					expect(actualConcerns).toEqual<Concerns>([])
 				})
 			},
 		)
 
 		describe("and the rule is disabled", () => {
+			const actualConcerns = useConciseSubjectLines([commit], null)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], null)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
@@ -257,16 +273,18 @@ describe.each`
 		`(
 			"and the rule is enabled with a maximum length of $options.maxLength characters",
 			(context: RuleContext<"useConciseSubjectLines">) => {
+				const actualConcerns = useConciseSubjectLines([commit], context.options)
+
 				it("does not raise any concerns", () => {
-					const actualConcerns = useConciseSubjectLines([commit], context.options)
 					expect(actualConcerns).toEqual<Concerns>([])
 				})
 			},
 		)
 
 		describe("and the rule is disabled", () => {
+			const actualConcerns = useConciseSubjectLines([commit], null)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines([commit], null)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		})
@@ -299,8 +317,9 @@ describe("when verifying a set of multiple commits and some commits have long su
 	]
 
 	describe("and the rule is enabled with a maximum length of 20 characters", () => {
+		const actualConcerns = useConciseSubjectLines(commits, enabled20.options)
+
 		it("raises concerns about the commits whose subject lines exceed 20 characters", () => {
-			const actualConcerns = useConciseSubjectLines(commits, enabled20.options)
 			expect(actualConcerns).toEqual<Concerns>([
 				subjectLineConcern(enabled20, commits[0].sha, { range: [20, 52] }),
 				subjectLineConcern(enabled20, commits[1].sha, { range: [20, 24] }),
@@ -315,8 +334,9 @@ describe("when verifying a set of multiple commits and some commits have long su
 	})
 
 	describe("and the rule is enabled with a maximum length of 50 characters", () => {
+		const actualConcerns = useConciseSubjectLines(commits, enabled50.options)
+
 		it("raises concerns about the commits whose subject lines exceed 50 characters", () => {
-			const actualConcerns = useConciseSubjectLines(commits, enabled50.options)
 			expect(actualConcerns).toEqual<Concerns>([
 				subjectLineConcern(enabled50, commits[0].sha, { range: [50, 52] }),
 				subjectLineConcern(enabled50, commits[7].sha, { range: [50, 75] }),
@@ -326,8 +346,9 @@ describe("when verifying a set of multiple commits and some commits have long su
 	})
 
 	describe("and the rule is enabled with a maximum length of 72 characters", () => {
+		const actualConcerns = useConciseSubjectLines(commits, enabled72.options)
+
 		it("raises concerns about the commits whose subject lines exceed 72 characters", () => {
-			const actualConcerns = useConciseSubjectLines(commits, enabled72.options)
 			expect(actualConcerns).toEqual<Concerns>([
 				subjectLineConcern(enabled72, commits[7].sha, { range: [72, 75] }),
 				subjectLineConcern(enabled72, commits[11].sha, { range: [72, 76] }),
@@ -336,8 +357,9 @@ describe("when verifying a set of multiple commits and some commits have long su
 	})
 
 	describe("and the rule is disabled", () => {
+		const actualConcerns = useConciseSubjectLines(commits, null)
+
 		it("does not raise any concerns", () => {
-			const actualConcerns = useConciseSubjectLines(commits, null)
 			expect(actualConcerns).toEqual<Concerns>([])
 		})
 	})
@@ -359,16 +381,18 @@ describe("when verifying a set of multiple commits and all commits have concise
 	`(
 		"and the rule is enabled with a maximum length of $options.maxLength characters",
 		(context: RuleContext<"useConciseSubjectLines">) => {
+			const actualConcerns = useConciseSubjectLines(commits, context.options)
+
 			it("does not raise any concerns", () => {
-				const actualConcerns = useConciseSubjectLines(commits, context.options)
 				expect(actualConcerns).toEqual<Concerns>([])
 			})
 		},
 	)
 
 	describe("and the rule is disabled", () => {
+		const actualConcerns = useConciseSubjectLines(commits, null)
+
 		it("does not raise any concerns", () => {
-			const actualConcerns = useConciseSubjectLines(commits, null)
 			expect(actualConcerns).toEqual<Concerns>([])
 		})
 	})
diff --git a/src/domains/rules/concerns/CommitConcern.ts b/src/domains/rules/concerns/CommitConcern.ts
new file mode 100644
index 00000000..c0c31ffe
--- /dev/null
+++ b/src/domains/rules/concerns/CommitConcern.ts
@@ -0,0 +1,12 @@
+import type { RuleContext } from "#rules/Rule.ts"
+import type { CommitSha } from "#types/CommitSha.ts"
+
+export type CommitConcern = {
+	location: "commit"
+	rule: RuleContext
+	commitSha: CommitSha
+}
+
+export function commitConcern(rule: RuleContext, commitSha: CommitSha): CommitConcern {
+	return { location: "commit", rule, commitSha }
+}
diff --git a/src/domains/rules/concerns/Concern.ts b/src/domains/rules/concerns/Concern.ts
index 00b0b5f8..bbbd03c0 100644
--- a/src/domains/rules/concerns/Concern.ts
+++ b/src/domains/rules/concerns/Concern.ts
@@ -3,6 +3,7 @@ import type { Configuration } from "#configurations/Configuration.ts"
 import type { AuthorEmailAddressConcern } from "#rules/concerns/AuthorEmailAddressConcern.ts"
 import type { AuthorNameConcern } from "#rules/concerns/AuthorNameConcern.ts"
 import type { BodyLineConcern } from "#rules/concerns/BodyLineConcern.ts"
+import type { CommitConcern } from "#rules/concerns/CommitConcern.ts"
 import type { CommitterEmailAddressConcern } from "#rules/concerns/CommitterEmailAddressConcern.ts"
 import type { CommitterNameConcern } from "#rules/concerns/CommitterNameConcern.ts"
 import type { SubjectLineConcern } from "#rules/concerns/SubjectLineConcern.ts"
@@ -32,6 +33,7 @@ export type Concern =
 	| AuthorEmailAddressConcern
 	| AuthorNameConcern
 	| BodyLineConcern
+	| CommitConcern
 	| CommitterEmailAddressConcern
 	| CommitterNameConcern
 	| SubjectLineConcern
diff --git a/src/domains/rules/reports/CommitwiseReport.tests.ts b/src/domains/rules/reports/CommitwiseReport.tests.ts
index b6d5d008..0fb128fa 100644
--- a/src/domains/rules/reports/CommitwiseReport.tests.ts
+++ b/src/domains/rules/reports/CommitwiseReport.tests.ts
@@ -2,14 +2,15 @@ import { describe, expect, it } from "vitest"
 import { fakeCommitFactory } from "#commits/Commit.fixtures.ts"
 import type { Commit, Commits } from "#commits/Commit.ts"
 import { fakeConfiguration } from "#configurations/Configuration.fixtures.ts"
+import { commitConcern } from "#rules/concerns/CommitConcern.ts"
 import type { Concerns } from "#rules/concerns/Concern.ts"
 import { subjectLineConcern } from "#rules/concerns/SubjectLineConcern.ts"
 import { commitwiseReport } from "#rules/reports/CommitwiseReport.ts"
 import { ruleContext } from "#rules/Rule.ts"
+import { fakeCommitSha } from "#types/CommitSha.fixtures.ts"
 import type { Vector } from "#types/Vector.ts"
 
-const configuration = fakeConfiguration()
-const fakeCommit = fakeCommitFactory(configuration)
+const fakeCommit = fakeCommitFactory(fakeConfiguration())
 
 describe("when there are no concerns", () => {
 	const commits: Commits = []
@@ -21,7 +22,49 @@ describe("when there are no concerns", () => {
 	})
 })
 
-describe("when 'noRevertRevertCommits' has a concern about characters 0-16", () => {
+describe("when 'noMergeCommits' has a concern about the commit", () => {
+	const commit = fakeCommit({
+		sha: "507c835ff93e38ed1540ff58fb72f7837f9af13",
+		parents: [fakeCommitSha(), fakeCommitSha()],
+		message: "Merge branch 'main' into bugfix/dance-party-playlist",
+	})
+	const concern = commitConcern(ruleContext("noMergeCommits"), commit.sha)
+
+	it("describes the rule violation", () => {
+		const actualOutput = commitwiseReport([commit], [concern])
+		expect(actualOutput).toBe(
+			`
+507c835 Merge branch 'main' into bugfix/dance-party-playlist
+      ╭─────────────────────────────────────────────────────
+      ╰─ Merge commits are not allowed.
+         (noMergeCommits)
+`.trim(),
+		)
+	})
+})
+
+describe("when 'noMergeCommits' has a concern about the commit", () => {
+	const commit = fakeCommit({
+		sha: "71516e17c94c69de4eeafff60fac072764d2",
+		parents: [fakeCommitSha(), fakeCommitSha(), fakeCommitSha()],
+		message: "amend! Merge branch 'feature/new-coffee-machine' into feature/office-overhaul",
+	})
+	const concern = commitConcern(ruleContext("noMergeCommits"), commit.sha)
+
+	it("describes the rule violation", () => {
+		const actualOutput = commitwiseReport([commit], [concern])
+		expect(actualOutput).toBe(
+			`
+71516e1 amend! Merge branch 'feature/new-coffee-machine' into feature/office-overhaul
+      ╭──────────────────────────────────────────────────────────────────────────────
+      ╰─ Merge commits are not allowed.
+         (noMergeCommits)
+`.trim(),
+		)
+	})
+})
+
+describe("when 'noRevertRevertCommits' has a concern about characters 0-16 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "d4e7a978cea34b727ea52f90c928fd535e4aee",
 		message: 'Revert "Revert "Make the program act like a clown""',
@@ -43,7 +86,7 @@ d4e7a97 Revert "Revert "Make the program act like a clown""
 	})
 })
 
-describe("when 'noRevertRevertCommits' has a concern about characters 1-26", () => {
+describe("when 'noRevertRevertCommits' has a concern about characters 1-26 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "34aa41b818c40682cabeecd5623dfe51df7a4a5",
 		message: ' revert "revert  "revert "repair the soft ice machine """',
@@ -65,7 +108,7 @@ describe("when 'noRevertRevertCommits' has a concern about characters 1-26", ()
 	})
 })
 
-describe("when 'noSquashMarkers' has a concern about characters 0-6", () => {
+describe("when 'noSquashMarkers' has a concern about characters 0-6 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "ffebad193fe7d02aa9b19b70ee132a26f14f8caf",
 		message: "amend!Apply strawberry jam to make the code sweeter",
@@ -85,7 +128,7 @@ ffebad1 amend!Apply strawberry jam to make the code sweeter
 	})
 })
 
-describe("when 'noSquashMarkers' has a concern about characters 1-14", () => {
+describe("when 'noSquashMarkers' has a concern about characters 1-14 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "56c750b0811fbcad2b237b2b99fc7d3fc91b926",
 		message: " fixup! fixup! found a funny easter egg",
@@ -107,7 +150,7 @@ describe("when 'noSquashMarkers' has a concern about characters 1-14", () => {
 	})
 })
 
-describe("when 'useCapitalisedSubjectLines' has a concern about characters 0-1", () => {
+describe("when 'useCapitalisedSubjectLines' has a concern about characters 0-1 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "497de39943643a56f7a69d3d19723e3035318644",
 		message: "release the robot butler",
@@ -129,7 +172,7 @@ describe("when 'useCapitalisedSubjectLines' has a concern about characters 0-1",
 	})
 })
 
-describe("when 'useCapitalisedSubjectLines' has a concern about characters 7-8", () => {
+describe("when 'useCapitalisedSubjectLines' has a concern about characters 7-8 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "92d6b11650c6b63d64fd77522241b7f50ff5b",
 		message: "fixup! resolve a bug that thought it was a feature",
@@ -151,7 +194,7 @@ describe("when 'useCapitalisedSubjectLines' has a concern about characters 7-8",
 	})
 })
 
-describe("when 'useConciseSubjectLines' has a concern about characters 20-25", () => {
+describe("when 'useConciseSubjectLines' has a concern about characters 20-25 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "68e921648c4a19e93d72f42a5d39c3eba704e41",
 		message: "Remove redundant call to `wrapper`",
@@ -175,7 +218,7 @@ describe("when 'useConciseSubjectLines' has a concern about characters 20-25", (
 	})
 })
 
-describe("when 'useConciseSubjectLines' has a concern about characters 20-67", () => {
+describe("when 'useConciseSubjectLines' has a concern about characters 20-67 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "9bed522bd48f0aee7574635bb23f5decdc4999",
 		message: "revisit the boolean properties in the `IceCreamMachine` constructor",
@@ -199,7 +242,7 @@ describe("when 'useConciseSubjectLines' has a concern about characters 20-67", (
 	})
 })
 
-describe("when 'useConciseSubjectLines' has a concern about characters 50-52", () => {
+describe("when 'useConciseSubjectLines' has a concern about characters 50-52 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "e8c95d69587a51685070837aaf3a8746e3cbba8",
 		message: "Retrieve data from the exclusive third-party service",
@@ -223,7 +266,7 @@ e8c95d6 Retrieve data from the exclusive third-party service
 	})
 })
 
-describe("when 'useConciseSubjectLines' has a concern about characters 72-76", () => {
+describe("when 'useConciseSubjectLines' has a concern about characters 72-76 of the subject line", () => {
 	const commit = fakeCommit({
 		sha: "be86674322213fb408d176589fadbcd44a2df",
 		message: "make a genuine attempt to fix the bugs that the users were complaining about",
diff --git a/src/domains/rules/reports/CommitwiseReport.ts b/src/domains/rules/reports/CommitwiseReport.ts
index cc2b08fe..61850456 100644
--- a/src/domains/rules/reports/CommitwiseReport.ts
+++ b/src/domains/rules/reports/CommitwiseReport.ts
@@ -1,6 +1,8 @@
 import type { Commit, Commits } from "#commits/Commit.ts"
 import { formatTokenisedLine } from "#commits/tokens/Token.ts"
+import type { CommitConcern } from "#rules/concerns/CommitConcern.ts"
 import { type Concern, type Concerns, concernedCommit } from "#rules/concerns/Concern.ts"
+import type { SubjectLineConcern } from "#rules/concerns/SubjectLineConcern.ts"
 import type { RuleContext } from "#rules/Rule.ts"
 import { formatCharacterRange } from "#types/CharacterRange.ts"
 import { indentString } from "#utilities/Strings.ts"
@@ -11,12 +13,57 @@ export function commitwiseReport(commits: Commits, concerns: Concerns): string {
 		.join("\n\n")
 }
 
+function formatConcern(concern: Concern, commit: Commit): string {
+	switch (concern.location) {
+		case "author-email-address": {
+			throw new Error(`Not implemented yet: ${concern.location}`)
+		}
+		case "author-name": {
+			throw new Error(`Not implemented yet: ${concern.location}`)
+		}
+		case "body-line": {
+			throw new Error(`Not implemented yet: ${concern.location}`)
+		}
+		case "commit": {
+			return formatCommitConcern(concern, commit)
+		}
+		case "committer-email-address": {
+			throw new Error(`Not implemented yet: ${concern.location}`)
+		}
+		case "committer-name": {
+			throw new Error(`Not implemented yet: ${concern.location}`)
+		}
+		case "subject-line": {
+			return formatSubjectLineConcern(concern, commit)
+		}
+	}
+}
+
 const SHORT_SHA_LENGTH = 7
 
+const RANGE_PREFIX = "╭─"
 const MESSAGE_PREFIX = "╰─"
 const MESSAGE_SUFFIX = "─╯"
 
-function formatConcern(concern: Concern, commit: Commit): string {
+function formatCommitConcern(concern: CommitConcern, commit: Commit): string {
+	const formattedSubjectLine = formatTokenisedLine(commit.subjectLine)
+	const commitLine = `${commit.sha.slice(0, SHORT_SHA_LENGTH)} ${formattedSubjectLine}`
+	const message = ruleMessage(concern.rule)
+
+	const rangeLine = indentString(
+		RANGE_PREFIX + "─".repeat(formattedSubjectLine.length),
+		SHORT_SHA_LENGTH - RANGE_PREFIX.length + 1,
+	)
+
+	const messageLine = indentString(
+		`${MESSAGE_PREFIX} ${message}\n   (${concern.rule.key})`,
+		SHORT_SHA_LENGTH - MESSAGE_PREFIX.length + 1,
+	)
+
+	return `${commitLine}\n${rangeLine}\n${messageLine}`
+}
+
+function formatSubjectLineConcern(concern: SubjectLineConcern, commit: Commit): string {
 	const commitLine = `${commit.sha.slice(0, SHORT_SHA_LENGTH)} ${formatTokenisedLine(commit.subjectLine)}`
 
 	const [rangeStart, rangeEnd] = concern.range
@@ -50,7 +97,7 @@ function ruleMessage(rule: RuleContext): string {
 			throw new Error(`Not implemented yet: ${rule.key}`)
 		}
 		case "noMergeCommits": {
-			throw new Error(`Not implemented yet: ${rule.key}`)
+			return "Merge commits are not allowed."
 		}
 		case "noRepeatedSubjectLines": {
 			throw new Error(`Not implemented yet: ${rule.key}`)