From where did the @ sign originate?

"Linguists are divided. Some think it originated in the early Middle Ages, when monks laboring over manuscripts contracted the versatile Latin word "ad" - which can mean "at" or "towards" or "by" - into a single character. Most linguists, however, say that the @ sign is a more recent invention, appearing sometime during the 18th century as a commercial symbol indicating price per unit, as in "5 apples @ 10 pence." Yet another linguist, researcher Denis Muzerelle, says the sign is the result of a different twist, when the accent over the word "à" used by French and German merchants was hastily extended.

But last July an Italian researcher discovered some 14th-century Venetian commercial documents clearly marked by the @ sign, where it was used to represent a gauge of quantity, the "anfora," or jar. Giorgio Stabile also found a Latin-Spanish dictionary dating from 1492 where "anfora" is translated into "arroba," a measure of weight. It's therefore natural that, in 1885 the "commercial a" was included on the keyboard of the first model of Underwood typewriter and from there migrated into the standard set of computing characters (such as ASCII) 80 years later.

The biggest problem with the @ sign nowadays is what to call it. Spaniards and Portuguese still use "arroba" -- which the French have borrowed and turned into "arobase." Americans and Britons call it the "at-sign." So do the Germans ("at-Zeichen"), Estonians ("ät-märk") and Japanese ("atto maak")."

Reference: http://answers.yahoo.com/question/index?qid=20060922101214AAIvRSg