At sign

Shopkeepers and clerks used @ to price goods by the unit: 5 apples @ 2p meant two pence for each apple. That bookkeeping habit is how it reached typewriter keyboards, and Unicode’s formal name for it, commercial at, still remembers it.

In 1971 Ray Tomlinson needed something to divide a user’s name from the computer’s name in the first email sent across a network. He chose @ because it was already on his keyboard and turned up in nobody’s name, so an address could never be misread. Spanish and Portuguese speakers call it arroba, also the name of an old measure of weight.