En dash / em dash
Use the shorter en dash for ranges and relationships; use the longer em dash for a break or interruption inside a sentence.
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What it is
An en dash, Unicode U+2013, connects a span or relationship: page 12 through page 18, the London to Paris route, or a score with two sides. An em dash, U+2014, cuts into a sentence for a break, interruption, or sharp turn. Their names come from the traditional widths of the letters n and m, though modern fonts do not have to make those widths exact.
Reach for the en dash wherever 'to' or 'through' could replace the mark. Reach for the em dash where commas feel weak and parentheses feel too quiet. The Chicago Manual of Style sets the em dash closed up against the words on either side; AP style puts a space on each side, so follow the publication in front of you. On a Mac, Option and hyphen gives the en dash and Option, Shift, and hyphen gives the em dash; on Windows the numeric-keypad codes are Alt+0150 and Alt+0151. One more consideration since 2023: a page full of unspaced em dashes now reads to many people as machine-written, so if your copy leans on them, count them.
The gotcha is asking one keyboard hyphen to do four jobs. A hyphen joins words, an en dash spans values, an em dash breaks a sentence, and a minus sign is mathematics. They have different Unicode code points and different shapes. Do not fake a long dash with two hyphens in finished copy.
Ask AI for it
Typeset this English manuscript with distinct Unicode punctuation. Use U+2013, or the HTML entity –, for closed ranges and paired relationships. Use U+2014, or —, for sentence breaks and interruptions. Keep U+002D for compound-word hyphens and U+2212 for mathematical minus signs. Follow Chicago Manual of Style spacing with no spaces around the em dash, but do not rewrite dashes inside code, URLs, command-line flags, ISBNs, or negative numbers. Return the edited copy plus a list of every replacement by line.