From our colleagues over at The Lede, The New York Times’s news blog:

Because it is used in every e-mail address and many tweets, you might be forgiven for thinking that the remarkably common symbol @, which English-speakers know as the “at sign,” but Italians call a “snail,” and south Slavs know as a “monkey,” is a fairly recent invention. In fact, as Wired magazine’s Tony Long points out, a Florentine merchant named Francesco Lapi used the symbol @ in a letter written 473 years ago today, on May 4, 1536.

The Italian newspaper La Repubblica reported in 2000 that Giorgio Stabile, who was then a professor of the history of science @ La Sapienza University in Rome, had come across the symbol in the merchant’s letter, where it was used to indicate an ancient measure of weight or volume, an amphora.

As Mr. Stabile explained to The Guardian in 2000, Francesco Lapi’s letter was sent from Seville to Rome and described the cargo on three ships that had just returned to Spain from Latin America:

“There, an amphora of wine, which is one thirtieth of a barrel, is worth 70 or 80 ducats,” Mr. Lapi informs his correspondent, representing the amphora with the now familiar symbol of an “a” wrapped in its own tail.

The Spanish word for amphora was “arroba,” and the Oxford English Dictionary explains that the unit was approximately 25 pounds of a solid or about 3 gallons of a liquid. In modern Spanish, the @ symbol on keyboards isstill called an arroba — as a Google image search illustrates. The word “arroba” itself was a Spanish corruption of an older Arabic word.

The symbol ended up on typewriter keyboards after it evolved over the centuries into commercial accounting shorthand for the phrase “at the price of” in records of transactions written by English merchants.

That’s why the symbol was sitting on a computer keyboard in 1971 when an engineer named Ray Tomlinson decided to use it in the first e-mail address to send the first e-mail. As Mr. Tomlinson himself has explained in a description of that first e-mail:

I chose to append an at sign and the host name to the user’s (login) name. I am frequently asked why I chose the at sign, but the at sign just makes sense. The purpose of the at sign (in English) was to indicate a unit price (for example, 10 items @ $1.95). I used the at sign to indicate that the user was “at” some other host rather than being local.

In case you’re wondering, Mr. Tomlinson says that he has no idea what the first few successfully transmitted e-mail messages said:

I sent a number of test messages to myself from one machine to the other. The test messages were entirely forgettable and I have, therefore, forgotten them.