in the early 1960s, my mother instructed me in letter-writing etiquette. After my signature, she told me, I was to add the symbols “x” and “o.” A kiss and a hug.

The art of writing longhand may have faded, but many of us continue to emit x’s and o’s like a binary love code in the e-mails that consume our daily lives.

Where do those symbols come from, these ur-emoticons that we sprinkle so liberally across our correspondence?

The Internet abounds with origin theories. There are visual explanations: that “x” resembles a kiss, for example; that “o” looks like an embrace or the union of bodies; and that “x” and “o” together form a kiss on a face. Then there are auditory explanations, such as the similarity in the pronunciation of “x” and “kiss.”

There is no definitive answer to how a cross came to mean a kiss, but it’s most likely to have evolved from the written tradition. The symbol x is the letter taw in early Hebrew (and in Ezekiel, a mark set “upon the foreheads” of men) and chi in Greek.

When Christianity came along, x came to represent a cross. “X meant Christ, and because of that, it meant faith and fidelity,” says Marcel Danesi, a professor of linguistic anthropology and semiotics at the University of Toronto. “We still see it on churches from medieval times.”

The x became the signature of choice in the Middle Ages, a time when few people could write, and documents were sealed with an x embossed in wax or lead. At the same time, letters and books, as well as oaths of political and economic fealty between kings and their vassals, were “sealed with a kiss” — an early antecedent of the acronym SWAK, which became popular during World War I for soldiers to imprint on their letters home.

“Symbols have a way of jumping from one domain to another,” says Danesi, who wrote “The History of a Kiss: The Birth of Popular Culture.” And it’s a small step to come from sealing a letter to sealing a love affair. He speculates that “x” underwent a conversion in an act of medieval romantic rebellion. “Romantic love becomes an obsession, and the kiss became empowering. It said to family and society: ‘You can’t tell me who I should marry.’ ”

The Oxford English Dictionary attributes the first recorded use of x as a kiss to British curate and naturalist Gilbert White in a 1763 letter which ended, “I am with many a xxxxxxx and many a Pater noster and Ave Maria, Gil White.”

Stephen Goranson, a researcher at Duke University, disagrees with the OED. “The x’s in White’s letter could possibly mean kisses, but it is more likely they meant blessings,” he says. Their juxtaposition with “Ave Maria” is similar to Daniel Defoe’s use of x in “Robinson Crusoe,” which was published in 1719 and refers to crosses as blessings. Goranson prefers a later OED citation, an 1894 letter by Winston Churchill to his mama. “Please excuse bad writing as I am in an awful hurry. (Many kisses.) xxx WSC.”

In his research, Goranson found several citations from 1880 on, such as a poem published in 1893: “Why do our sweet sentimental young misses / In love letters make little crosses for kisses?”

Goranson adds that blessings and kisses have been intertwined for all of history. “Mystics went back and forth on the love of God and love of a beloved spouse going way back,” he says. “Just look at ‘The Song of Songs.’ The same song could be one person’s devotional hymn and another’s love poem.”

Less is known about how “o” came to signify a hug.

An oft-quoted and unconfirmed “o” theory was postulated by the late Leo Rosten in his 1968 “Joys of Yiddish.” Rosten suggested that the “o” may have evolved also as a signature. When Jewish immigrants who could not write in Latin script arrived at Ellis Island, they refused to sign entry forms with the customary “x,” which they interpreted as a crucifix and a symbol of oppression. Instead they drew an “o,” leading immigration inspectors to call anyone who signed with an “o” “a kikel [circle in Yiddish] or kikeleh [little circle], which was shortened to kike,” and eventually took on a derogatory meaning.

When “o” joined “x” is another big question.

There’s the tic-tac-toe theory. The game, which has roots in ancient Egypt and Rome, was played with pebbles or coins until it moved to paper. “These are two of the simplest contrasting symbols, easy to master by illiterate people,” says David Parlett, author of “The Oxford History of Board Games.” He remains unsure how tic-tac-toe’s cross and circle could have metamorphosed into hugs and kisses.

The earliest mention of “x” and “o” together that Goranson found was in a letter to the Fort Pierce News Tribune in Florida, dated Nov. 22, 1960. “Dear Santa, How are you? I am fine. . . . Will you please bring me a play rifle and . . . please Love & Kisses XOXOXO DAVY MIKEY & CHERYL.”

And in a discussion chain on the American Dialect Society, linguist Ben Zimmer, in a search of newspaper archives, found “xoxo” and “xoxoxo” used in personal ads from about 1972.

“X” and “o” continue to flourish alongside countless newly minted emoticons. Diane Sawyer and Arianna Huffington sign their e-mails with “xo.” Critics deride the usage as girlish, while defenders point out that “x” and “o” are useful tools for women to soften direct requests, similar to how men traditionally use humor.

“ ‘X’ and ‘o’ are part of a whole array of symbols among girls and women that have one thing in common: They are expressive,” says Deborah Tannen, professor of linguistics at Georgetown University and author of “You Just Don’t Understand: Women and Men in Conversation.” These include exclamation points and the use of capital letters. “Girls and women have to show a certain level of enthusiasm to seem sincere,” she says. “If they don’t, it comes across to other girls and women as cold or rejecting.”

Will “x” and “o” be picked up by new generations? “Today 15-year-old girls around world have new emoticons for love,” says Scott Fahlman, a Carnegie Mellon professor of language technologies and computer science, who is credited with inventing the original smiley face emoticon, :), to denote sarcasm. “One version is a <3 forming a heart.”

Standard alphabetical characters, however, are being replaced by computer-generated graphics. And soon, Fahlman says, these graphic emoticons may be replaced by video images.

I intend to stick with“x” and “o,” which I will always associate with the warmth and love of my mom. They are a way to pass that affection on. XO!

Epstein is the editor of Moment Magazine.