Command

a road sign on a keyboard

Unicode: U+2318 place of interest sign
Meaning: On a Macintosh keyboard, the modifier key. Everywhere else, and for far longer, a waymark: something of interest here.
History: Susan Kare drew it into the Macintosh interface in 1983–84, after Steve Jobs objected to the Apple logo appearing beside every menu command. She took the looped square from a symbol reference: it is the Nordic sign for a place of cultural interest, itself an old form known as Saint John’s Arms. Unicode’s name for the character preserves the older job rather than the famous one — the codepoint is the provenance.
Variants: Drawn with square or rounded loops; the road-sign version and the keycap version differ in weight.
Related: ⏻ ☰ marks that outlived or escaped their first machine (→ Visual Grammar).
Appears: Swedish and Finnish road signs, campground and heritage maps, and roughly every Mac keyboard shipped since 1984.
Evolution: Nordic mark → tourist waymark → computer modifier key, with the road sign still in service and unaware.

Sources · 3

documented Susan Kare’s accounts of the Macintosh icon and keyboard work, including the origin of the substitution.
documented The Unicode Standard, U+2318 PLACE OF INTEREST SIGN — the name itself is the citation.
traditional Saint John’s Arms as a protective or apotropaic mark in Nordic folk practice before its cartographic use. Widely attested in folklore collections; the line from that use to the road sign is not documented.