em quad
Definition - A printer's term for a space one em wide, i.e. the width of the letter m.

Etymology -
The quad is a small block of metal, lower than the face of the type, used by printers for spacing. The em quad was so named because it was the same width as the letter m.

Quotation -
The em quad and the em space are usually synonymous as they both start as a space with a width of 1 em. However, in electronic publishing a few contrast the two by holding that if the font is condensed or expanded to alter the width of the characters, the em quad remains 1 em in width, while the em space is altered in width to the same proportion as the printing characters.
(Source: Wictionary s.v. em quad )

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