diff --git a/src/markdown/commonmark/grammar.ts b/src/markdown/commonmark/grammar.ts
index 17b776c..cd4aca9 100644
--- a/src/markdown/commonmark/grammar.ts
+++ b/src/markdown/commonmark/grammar.ts
@@ -203,7 +203,7 @@ export function holdsThematicBreak(tail: ThematicBreakTail | undefined, text: st
   return text.length >= tail.shortest && text.length <= tail.longest && text.charAt(0) === tail.marker
 }
 
-// Every line isThematicBreak accepts passes this, and a joined line passes exactly when each part does, so BlockText derives it from a prefix and its body.
+// Every line isThematicBreak accepts passes this.
 export function holdsOnlyRuleCharacters(text: string): boolean {
   for (let index = 0; index < text.length; index += 1) {
     const character = text.charAt(index)
diff --git a/src/markdown/emit/adf-to-markdown.ts b/src/markdown/emit/adf-to-markdown.ts
index a704f58..e9af1b5 100644
--- a/src/markdown/emit/adf-to-markdown.ts
+++ b/src/markdown/emit/adf-to-markdown.ts
@@ -3,13 +3,13 @@ import type { BlockNodeModel } from '../../adf/block-nodes.ts'
 import type { WrittenFlavour } from '../flavour.ts'
 import { blockDirectiveForm, documentSpelling } from '../directive/block.ts'
 import { blockNodeModel, blockNodes } from '../../adf/block-nodes.ts'
-import { blockText, emptyLine, joinedText, prefixedText, quotedText, writtenText, type BlockText } from './block-text.ts'
+import { blockText, emptyLine, holdsBlankLine, joinedText, opensWithThematicBreak, prefixedText, quotedText, writtenText, type BlockText } from './block-text.ts'
 import { carriedBlock } from '../opaque-carry.ts'
 import { emitInlineLine } from './inline-line.ts'
 import { failure, success, type ConvertErrorPath, type Result } from '../../result.ts'
 import { fencedCodeBlock } from '../commonmark/backtick-runs.ts'
 import { flavourClaims } from '../flavour.ts'
-import { holdsNullCharacter, isThematicBreak, markerInterruptsParagraph } from '../commonmark/grammar.ts'
+import { holdsNullCharacter, markerInterruptsParagraph } from '../commonmark/grammar.ts'
 import { holdsOnlyAttributes, isUnmarkedSpellableText, nodeAttrs, nodeContent, writableContent } from '../../adf/document.ts'
 import { languageSlot, type LanguageSlot } from '../code-language.ts'
 import { largestNesting } from '../../nesting.ts'
@@ -309,12 +309,12 @@ function listStart(node: AdfNode, items: number): number | undefined {
 
 function tryListItemLines(inner: BlockText, marker: string): BlockText | undefined {
   if (inner.length === 0) return blockText(marker.trimEnd())
-  if (inner.firstBlank || inner.blankRest > 0) return undefined
+  if (holdsBlankLine(inner)) return undefined
   const indent = ' '.repeat(marker.length)
   const lines = prefixedText(marker, indent, inner)
-  if (!lines.firstRuleCharacters || !isThematicBreak(lines.first)) return lines
+  if (!opensWithThematicBreak(lines)) return lines
   // An item opens with `*` only as text, a list there taking `- ` and a rule `---`; a backslash keeps a first line reading as a rule text.
-  return inner.firstCharacter === '*' ? prefixedText(`${marker}\\`, indent, inner) : undefined
+  return inner.first.character === '*' ? prefixedText(`${marker}\\`, indent, inner) : undefined
 }
 
 function tryParagraph(node: AdfNode, path: ConvertErrorPath, flavour: WrittenFlavour): Result<EmittedBlock> | undefined {
diff --git a/src/markdown/emit/block-text.test.ts b/src/markdown/emit/block-text.test.ts
index 97cb302..4b0f840 100644
--- a/src/markdown/emit/block-text.test.ts
+++ b/src/markdown/emit/block-text.test.ts
@@ -2,8 +2,8 @@ import assert from 'node:assert/strict'
 import fc from 'fast-check'
 import test from 'node:test'
 
-import { blockText, joinedText, prefixedText, quotedText, writtenText, type BlockText } from './block-text.ts'
-import { holdsOnlyRuleCharacters, isBlankLine } from '../commonmark/grammar.ts'
+import { blockText, holdsBlankLine, joinedText, opensWithThematicBreak, prefixedText, quotedText, writtenText, type BlockText } from './block-text.ts'
+import { holdsOnlyRuleCharacters, isBlankLine, isThematicBreak } from '../commonmark/grammar.ts'
 import { propertyRuns } from '../../conformance/property-harness.ts'
 
 const { tree } = fc.letrec<{ tree: BlockText }>((tie) => ({
@@ -20,15 +20,13 @@ test('measures a block text as the text it writes', () => {
   fc.assert(
     fc.property(tree, (text) => {
       const written = writtenText(text)
-      const [first = '', ...rest] = written.split('\n')
+      const [first = '', ...later] = written.split('\n')
+      const blank = (line: string): boolean => line !== '' && isBlankLine(line)
       assert.equal(text.length, written.length)
-      assert.equal(text.lineCount, rest.length + 1)
-      assert.equal(text.first, first)
-      assert.equal(text.firstCharacter, first.charAt(0))
-      assert.equal(text.firstBlank, first !== '' && isBlankLine(first))
-      assert.equal(text.firstRuleCharacters, holdsOnlyRuleCharacters(first))
-      assert.equal(text.blankRest, rest.filter((line) => line !== '' && isBlankLine(line)).length)
-      assert.equal(text.emptyRest, rest.filter((line) => line === '').length)
+      assert.deepEqual(text.first, { blank: blank(first), character: first.charAt(0), ruleCharacters: holdsOnlyRuleCharacters(first), text: first })
+      assert.deepEqual(text.later, { blank: later.filter(blank).length, empty: later.filter((line) => line === '').length, written: later.filter((line) => line !== '').length })
+      assert.equal(holdsBlankLine(text), [first, ...later].some(blank))
+      assert.equal(opensWithThematicBreak(text), isThematicBreak(first))
     }),
     propertyRuns(2000),
   )
diff --git a/src/markdown/emit/block-text.ts b/src/markdown/emit/block-text.ts
index 21b5137..d533399 100644
--- a/src/markdown/emit/block-text.ts
+++ b/src/markdown/emit/block-text.ts
@@ -1,91 +1,102 @@
-import { holdsOnlyRuleCharacters, isBlankLine } from '../commonmark/grammar.ts'
+import { holdsOnlyRuleCharacters, isBlankLine, isThematicBreak } from '../commonmark/grammar.ts'
 
 // A block's markdown, its prefixes applied only when written out, so a nesting level costs its own blocks rather than the text below it.
-export type BlockText = Readonly<Lines & Shape>
+export type BlockText = Readonly<Lines & { first: FirstLine; later: LaterLines; length: number }>
 type Lines =
   | { body: BlockText; head: string; indent: string; kind: 'prefixed' }
   | { body: BlockText; kind: 'quoted' }
   | { kind: 'joined'; parts: readonly BlockText[] }
   | { kind: 'text'; lines: readonly string[] }
 type Prefix = Lines & { kind: 'prefixed' | 'quoted' }
-// Measures of the written text. The flags describe `first` so no caller reads it: reading a concatenated string copies it whole, once per nesting level.
-type Shape = { blankRest: number; emptyRest: number; first: string; firstBlank: boolean; firstCharacter: string; firstRuleCharacters: boolean; length: number; lineCount: number }
+// The written first line. The flags describe `text` so no caller reads it: reading a concatenated string copies it whole, once per nesting level.
+type FirstLine = Readonly<{ blank: boolean; character: string; ruleCharacters: boolean; text: string }>
+// How many written lines after the first are blank, empty, or hold text, blank ones included.
+type LaterLines = Readonly<{ blank: number; empty: number; written: number }>
 
 export const emptyLine = blockText('')
 
 export function blockText(text: string): BlockText {
   const [first = '', ...rest] = text.split('\n')
-  const blankRest = rest.filter((line) => line !== '' && isBlankLine(line)).length
-  const firstShape = { first, firstBlank: first !== '' && isBlankLine(first), firstCharacter: first.charAt(0), firstRuleCharacters: holdsOnlyRuleCharacters(first) }
-  return { ...firstShape, blankRest, emptyRest: rest.filter((line) => line === '').length, kind: 'text', length: text.length, lineCount: rest.length + 1, lines: [first, ...rest] }
+  const empty = rest.filter((line) => line === '').length
+  const later = { blank: rest.filter((line) => line !== '' && isBlankLine(line)).length, empty, written: rest.length - empty }
+  return { first: { blank: first !== '' && isBlankLine(first), character: first.charAt(0), ruleCharacters: holdsOnlyRuleCharacters(first), text: first }, kind: 'text', later, length: text.length, lines: [first, ...rest] }
 }
 
 export function joinedText(parts: readonly BlockText[]): BlockText {
   const [first, ...others] = parts
   if (first === undefined) return emptyLine
-  let { blankRest, emptyRest, length, lineCount } = first
+  let { blank, empty, written } = first.later
+  let { length } = first
   for (const part of others) {
-    blankRest += part.blankRest + (part.firstBlank ? 1 : 0)
-    emptyRest += part.emptyRest + (part.first === '' ? 1 : 0)
+    blank += part.later.blank + (part.first.blank ? 1 : 0)
+    empty += part.later.empty + (part.first.text === '' ? 1 : 0)
+    written += part.later.written + (part.first.text === '' ? 0 : 1)
     length += 1 + part.length
-    lineCount += part.lineCount
   }
-  const { firstBlank, firstCharacter, firstRuleCharacters } = first
-  return { blankRest, emptyRest, first: first.first, firstBlank, firstCharacter, firstRuleCharacters, kind: 'joined', length, lineCount, parts }
+  return { first: first.first, kind: 'joined', later: { blank, empty, written }, length, parts }
 }
 
 // head, never blank, leads the first line; indent, spaces alone, leads every later line holding text.
 export function prefixedText(head: string, indent: string, body: BlockText): BlockText {
-  const first = `${head}${body.first}`
-  const length = body.length + head.length + indent.length * (body.lineCount - 1 - body.emptyRest)
-  const firstRuleCharacters = holdsOnlyRuleCharacters(head) && body.firstRuleCharacters
-  return { blankRest: body.blankRest, body, emptyRest: body.emptyRest, first, firstBlank: false, firstCharacter: head.charAt(0), firstRuleCharacters, head, indent, kind: 'prefixed', length, lineCount: body.lineCount }
+  const length = body.length + head.length + indent.length * body.later.written
+  return { body, first: headedLine(head, body.first), head, indent, kind: 'prefixed', later: body.later, length }
 }
 
+// Each line takes `> `, an empty one `>`.
 export function quotedText(body: BlockText): BlockText {
-  const empty = body.emptyRest + (body.first === '' ? 1 : 0)
-  const first = body.first === '' ? '>' : `> ${body.first}`
-  return { blankRest: 0, body, emptyRest: 0, first, firstBlank: false, firstCharacter: '>', firstRuleCharacters: false, kind: 'quoted', length: body.length + 2 * body.lineCount - empty, lineCount: body.lineCount }
+  const head = body.first.text === '' ? '>' : '> '
+  const later = { blank: 0, empty: 0, written: body.later.empty + body.later.written }
+  return { body, first: headedLine(head, body.first), kind: 'quoted', later, length: body.length + head.length + 2 * body.later.written + body.later.empty }
+}
+
+// A line holds only rule characters exactly when each of its parts does.
+function headedLine(head: string, line: FirstLine): FirstLine {
+  return { blank: false, character: head.charAt(0), ruleCharacters: holdsOnlyRuleCharacters(head) && line.ruleCharacters, text: `${head}${line.text}` }
+}
+
+export function holdsBlankLine(text: BlockText): boolean {
+  return text.first.blank || text.later.blank > 0
+}
+
+export function opensWithThematicBreak(text: BlockText): boolean {
+  return text.first.ruleCharacters && isThematicBreak(text.first.text)
 }
 
 // The first line, and the line break after it where one follows: enough to read a marker opening the text.
 export function opening(text: BlockText): string {
-  return text.lineCount > 1 ? `${text.first}\n` : text.first
+  return text.later.empty + text.later.written > 0 ? `${text.first.text}\n` : text.first.text
 }
 
 export function writtenText(text: BlockText): string {
   const lines: string[] = []
-  // The open prefixes, outermost first, and where the quotes stand among them.
+  // The open prefixes, outermost first. A line starts every open prefix, so the unstarted ones are the innermost.
   const open: Prefix[] = []
-  const quotes: number[] = []
-  // A line starts every open prefix, so the unstarted ones are the innermost.
   let started = 0
-  const pending: (BlockText | 'leave')[] = [text]
+  // Each pending text with how many prefixes stay open around it.
+  const pending: { text: BlockText; within: number }[] = [{ text, within: 0 }]
   for (let next = pending.pop(); next !== undefined; next = pending.pop()) {
-    if (next === 'leave') {
-      if (open.pop()?.kind === 'quoted') quotes.pop()
-      started = Math.min(started, open.length)
-    } else if (next.kind === 'text') {
-      for (const line of next.lines) {
-        lines.push(prefixedLine(line, open, quotes.at(-1), started))
+    open.length = next.within
+    started = Math.min(started, next.within)
+    const current = next.text
+    if (current.kind === 'text') {
+      for (const line of current.lines) {
+        lines.push(prefixedLine(line, open, started))
         started = open.length
       }
-    } else if (next.kind === 'joined') for (const part of next.parts.slice().reverse()) pending.push(part)
+    } else if (current.kind === 'joined') for (const part of current.parts.slice().reverse()) pending.push({ text: part, within: open.length })
     else {
-      if (next.kind === 'quoted') quotes.push(open.length)
-      open.push(next)
-      pending.push('leave', next.body)
+      open.push(current)
+      pending.push({ text: current.body, within: open.length })
     }
   }
   return lines.join('\n')
 }
 
-// Innermost first: whether a line is empty when an outer prefix meets it depends on the prefixes inside.
-function prefixedLine(line: string, open: readonly Prefix[], innermostQuote: number | undefined, started: number): string {
-  // An empty line takes no indent, so where every prefix has started its walk begins at the innermost quote.
-  const walked = line === '' && started === open.length ? open.slice(0, (innermostQuote ?? -1) + 1) : open
-  return walked.reduceRight((written, prefix, depth) => {
+// Innermost first, since an empty line takes no indent and a quote turns it into `>`.
+function prefixedLine(line: string, open: readonly Prefix[], started: number): string {
+  return open.reduceRight((written, prefix, depth) => {
     if (prefix.kind === 'quoted') return written === '' ? '>' : `> ${written}`
-    return depth >= started ? `${prefix.head}${written}` : `${prefix.indent}${written}`
+    if (depth >= started) return `${prefix.head}${written}`
+    return written === '' ? '' : `${prefix.indent}${written}`
   }, line)
 }
diff --git a/src/markdown/portable/block-spelling.ts b/src/markdown/portable/block-spelling.ts
index e309ab2..878649a 100644
--- a/src/markdown/portable/block-spelling.ts
+++ b/src/markdown/portable/block-spelling.ts
@@ -12,8 +12,8 @@ import { nodeAttrs, nodeContent } from '../../adf/document.ts'
 
 const portableBlockWriters: FlavourBlockWriters = {
   // GitHub reads a task marker opening any item's first paragraph as a checkbox, whatever its siblings hold.
-  claimsItem: (text) => text.firstCharacter === markerOpening && leadingMarker(opening(text), readTaskMarker) !== undefined,
-  claimsQuote: (text) => text.firstCharacter === markerOpening && leadingMarker(opening(text), readAlertMarker) !== undefined,
+  claimsItem: (text) => leadsWithMarker(text, readTaskMarker),
+  claimsQuote: (text) => leadsWithMarker(text, readAlertMarker),
   spell: spellPortableBlock,
 }
 
@@ -30,6 +30,11 @@ function spellPortableBlock(node: AdfNode, path: ConvertErrorPath, depth: number
   return line.ok ? quotedBlocks(node, path, depth, writing, headed(`${foldedAlertMarker} ${line.value}`)) : line
 }
 
+// Reading a concatenated line copies it whole, so the first character is checked before the line is read.
+function leadsWithMarker(text: BlockText, read: (text: string) => { length: number } | undefined): boolean {
+  return text.first.character === markerOpening && leadingMarker(opening(text), read) !== undefined
+}
+
 function headed(head: string): (body: BlockText) => BlockText {
   return (body) => (body.length === 0 ? blockText(head) : joinedText([blockText(head), emptyLine, body]))
 }
diff --git a/src/markdown/portable/conventions.test.ts b/src/markdown/portable/conventions.test.ts
new file mode 100644
index 0000000..4a7c917
--- /dev/null
+++ b/src/markdown/portable/conventions.test.ts
@@ -0,0 +1,15 @@
+import assert from 'node:assert/strict'
+import fc from 'fast-check'
+import test from 'node:test'
+
+import { markerOpening, readAlertMarker, readTaskMarker } from './conventions.ts'
+import { propertyRuns } from '../../conformance/property-harness.ts'
+
+test('reads a task or alert marker only where the text opens with the marker opening', () => {
+  fc.assert(
+    fc.property(fc.string({ maxLength: 8, unit: fc.constantFrom(' ', '!', '+', '-', 'N', 'x', '[', '\\', ']') }), (text) => {
+      if (readTaskMarker(text) !== undefined || readAlertMarker(text) !== undefined) assert.equal(text.charAt(0), markerOpening)
+    }),
+    propertyRuns(2000),
+  )
+})