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    Hyphens, En Dashes, and the Double-Hyphen Prose Dash

    3 min read
    Typography Writing AI Design

    Today I learned what a prose dash is after asking a language model to remove em dashes from its output.

    I noticed em dashes after reading hundreds of AI-generated responses. Models insert them into every second paragraph to add pauses and mimic conversation. Readers recognize that cadence as machine text.

    I instructed the model to replace every em dash with a plain hyphen (-). The model returned double hyphens (--). Manuscript editors call two consecutive hyphens a “prose dash” or typewriter dash.

    The swap fixed nothing. The parenthetical clause stayed awkward regardless of the symbol around it.

    Structural Edits Beat Punctuation Swaps

    Mechanical typewriters lacked dedicated keys for en and em dashes. Writers typed two hyphens to tell print shops to set an em dash.

    Swapping an em dash for a double hyphen preserves the parenthetical detour:

    <!-- Raw model output (em dash slop) -->
The new compiler—despite initial doubts from the team—cut build times by 45%.

<!-- Model hyphen replacement (prose dash slop) -->
The new compiler--despite initial doubts from the team--cut build times by 45%.

<!-- Single hyphen replacement (broken compound) -->
The new compiler - despite initial doubts from the team - cut build times by 45%.

<!-- Fix 1: Delete the parenthetical detour -->
The new compiler cut build times by 45%.

<!-- Fix 2: Split into two direct sentences -->
The new compiler cut build times by 45%. The team doubted the approach at first.

    Two structural edits fix machine-generated text:

    1. Delete the detour: Drop the parenthetical aside when it adds no new fact.
    2. Split the sentence: Put the main fact in sentence one, and the qualification in sentence two.

    The Size Ladder

    The four primary marks scale by width:

    MarkWidthPrimary JobExample
    Hyphen (-)Shortest (1 char)Joins compound words, links prefixesmother-in-law, two-thirds
    En Dash (–)Medium (~width of “n”)Shows number/date ranges, connectionspages 330–339, 1980–1995
    Em Dash (—)Longest (~width of “m”)Sets off parenthetical asides and breaksShe arrived—without warning—at dawn.
    Prose Dash (--)2 ASCII hyphensTypewriter shorthand for em dashHe ran--fast--to the station.

    Rules for Each Mark

    Pick the mark with these rules:

    1. Use a Hyphen (-) to join words:

      • Compound nouns and adjectives: high-priority queue, full-time job
      • Prefixes before capitalized words: anti-American, pre-1990

    2. Use an En Dash (–) for ranges and multi-word compound modifiers:

      • Value ranges: 10–20 ms latency, May–August 2026
      • Paired relationships: client–server architecture, San Francisco–Tokyo flight
      • Compound modifiers where one element contains two words: New York–based team, post–World War II

    3. Use an Em Dash (—) or Double Hyphen (--) for deliberate pauses in human prose:

      • Closed style (Chicago): The result—clean code—proved worth the work.
      • Spaced en dash style (Oxford): The result – clean code – proved worth the work.

    Specialized Typographic Variants

    Specialized layouts use three specific marks:

    • Minus Sign (−, U+2212): Matches the width and height of a plus sign (+). A standard hyphen sits too low for math expressions like 5 − 3 = 2.
    • Figure Dash (‒, U+2012): Matches the width of numeric digits (0–9). It prevents jagged columns in phone numbers and data tables (555‒0199).
    • Non-Breaking Hyphen (‑, U+2011): Matches the hyphen glyph but prevents the browser from wrapping the line at that word boundary.

    Keyboard Shortcuts

    MarkmacOSWindows (Alt Code)HTML EntityUnicode
    Hyphen-Hyphen&#45;U+002D
    En DashOption + -Alt + 0150&ndash; (&#8211;)U+2013
    Em DashShift + Option + -Alt + 0151&mdash; (&#8212;)U+2014
    Minus SignCharacter ViewerAlt + 8722&minus; (&#8722;)U+2212
    Non-Breaking HyphenOption + Command + -Alt + 8209&#8209;U+2011

    What I Learned

    • Language models replace em dashes with double-hyphen prose dashes (--) when prompted for hyphens.
    • The term “prose dash” refers to the typewriter convention of typing two hyphens as shorthand for an em dash.
    • Swapping punctuation glyphs leaves weak sentence structure intact. Fixing AI slop requires deleting parenthetical detours or splitting sentences.
    • Use en dashes (–) for numeric ranges (10–20ms) and multi-word modifiers like New York–based.
    • Minus signs (U+2212) align with math operators, while standard hyphens align with lowercase letter centers.
    • Figure dashes (U+2012) maintain fixed column widths in numeric tables.
    • Non-breaking hyphens (&#8209;) prevent orphan prefixes during text wrap.

    References

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