Em Dash

About this character

The em dash is the longest of the horizontal dashes in common English typography. Its name comes from the unit of typographic measure called an em, defined as the width of the capital M in the type size being used — historically the widest letter in the alphabet. Cast as a piece of metal type, the em dash was a single sort one em wide. The character entered English prose in the 18th century, where writers like Sterne, Dickinson, and Austen used it to mark abrupt breaks, asides, attributions, and unfinished thoughts.

Three style guides set the convention for surrounding spaces. The Associated Press style and most British house styles set the dash with a space on each side: word — word. The Chicago Manual of Style sets it tight, with no spaces: word—word. The New York Times follows AP. Most fiction in the United States follows Chicago. There is no Unicode-level distinction between the two — the dash is the same codepoint either way; the spaces are the publisher's choice. (Many writers will use thin spaces, U+2009, on either side as a compromise.)

U+2014 is one of four dash-family codepoints that are routinely confused. U+2010 HYPHEN and U+002D HYPHEN-MINUS are the short marks used inside compound words. U+2013 EN DASH is half the em dash's width and is used for numeric ranges (pages 12–18) and conjoined names (Sapir–Whorf). U+2212 MINUS SIGN is the mathematical minus, slightly longer than a hyphen and at the correct vertical position to align with the plus sign. The em dash is none of these. Code formatters and most Markdown processors will autoconvert -- to an en dash and --- to an em dash; pasted text often arrives as a hyphen-minus by mistake. See the General Punctuation block for the full set.

From the Unicode data

Its East Asian Width is Ambiguous: it takes one column in Western contexts and two in East Asian legacy contexts, a common cause of misaligned tables in terminals.