Slate tech columnist Farhad Manjoo argued recently that the fax machine lives on largely because of our attachment to the written signature. Manjoo’s observation piqued the Explainer’s curiosity: When did scribbling your name on a piece of paper become a means of authentication?

A long time ago. Signatures on written transactions have been customary in Jewish communities since about thesecond century and among Muslims since the Hegira (the migration of Muhammad and his followers to Medina) in 622. In Europe, the signature dates to the sixth century. But it didn’t catch on widely there for another thousand-odd years, until the 16th and 17th centuries, when education and literacy were on the rise and more agreements were made in writing. In England, the 1677 Statute of Frauds—which stipulated that contracts must exist in writing and bear a signature—was pivotal. Signatures became a standard form of validating agreements—a practice that was also adopted in colonial America.

Between the sixth century, when signatures first appeared, and the 17th century, when signing became standard practice, Europeans used various customs in addition to signing to formalize contracts. * Wax seals bearing an impressed or embossed figure were common, particularly among the French, who brought the tradition to England during the Norman invasion. (Seals also appear in the Bible, and to this day sealing, not signing, is standard practice in China, Japan, and Korea.) One popular way to create these impressions was to press a signet ring into beeswax. Signet rings themselves were also used as validation: A king might, for example, dispatch a herald bearing an oral message to a foreign power, and give him the royal signet ring so that the message’s recipient would be confident of its origin.

Other, more casual agreements were also made in physical form. Cutting off a lock of hair and giving it to someone else was one way to seal a contract. Around the 13th century, agreements were sometimes marked with a slap, or some other traumatic act. The theory was that both parties would remember not only the injury but the accord that was reached on its infliction.

Though individuality and legibility seem as if they ought to be essential traits of a signature, that’s never really been the case. Starting in the 9th and 10th centuries, scribes validated documents using the sign of the cross. This practice may account for the custom among illiterates of signing an “x” in place of their name. By contrast, among the literate gentility, a signature was not meant to be easily discerned—an intricate and illegible signature, rather than a printed one, suggested an education in handwriting.

It’s also long been accepted that paying close attention to signatures isn’t a particularly good way to detect or rule out fraud. A 1772 case of money order forgery in England, for example, hinged more on the alleged forger’s erratic behavior than on the subtle differences in handwriting from one document to the next. To catch identity theft, modern-day credit card companies and banks pay less attention to their customers’ signatures than to their spending habits. (One enthusiastic prankster anecdotally demonstrates the shocking insignificance of signature consistency here.)

The laws about what constitutes a signature have responded to technological change over the past several centuries. The development of the printing press—as well as, on a smaller scale, a craze in the 1860s and ‘70s for owning a custom rubber stamp bearing a facsimile of your signature—forced courts in both Britain and the United States to decide whether printing or stamping your name constituted signing (generally speaking: yes). The telegram also posed a substantial challenge to existing law: The 1869 case of Howley v. Whipple in New Hampshire established that an agreement made by telegram constituted a signed contract. On the Internet we “sign” things all the time, often without realizing it. In the United States and Britain, judges have upheld that a name typed at the bottom of an e-mail can constitute a signature, and that an e-mail can constitute a binding agreement.

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Explainer thanks Brigitte Miriam Bedos-Rezak of New York University,Jane Caplan of the University of Oxford, and Stephen Mason, author of Electronic Signatures in Law.

Correction, March 25, 2011:The article originally suggested that Europeans did not use signatures in the period between the sixth century and the 17th century. In fact, signing was practiced along with other means of authenticating agreements during that time. (Return to the corrected sentence.)