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All sports have their own vocabularies, the shorthand lingo to communicate intricacies of rules and how play proceeds. But usually the scoring can at least be counted on to be fairly straightforward. Not so much for tennis.

For the unfamiliar, tennis starts with both players at zero, called love: “Love-all.” One person scores: 15 to love. The server’s score is said first, the receiver’s second. The other now scores, and they’re tied at “15-all.” The next point is 30, then 40, and the following point wins that game. If they tie at 40 it’s called a deuce. From that tie the next person to get a point has the advantage, but generally has to win by two points — that is, to score twice in a row — to win the game. And it doesn’t stop there. Six of these games make a set, and the set must be won by two games or it goes to a tiebreaker. After the set is over, it repeats. To win the whole match requires either winning best of five sets or best of three sets, depending on the competition.

As tennis fans get ready to watch stars like Serena Williams, Roger Federer, Novak Djokovic and Naomi Osaka play at the U.S. Open, those less familiar with the game may once again ponder an inevitable question: Why count this way?

Disappointingly, the origins of pretty much every part of the scoring system are a mystery. “I don’t think anybody really knows how it started or why it developed how it did,” says Elizabeth Wilson, who wrote Love Game: A History of Tennis, from Victorian Pastime to Global Phenomenon.

“There are various theories, all sorts of romantic theories have been built up about it.” says Wilson. “That’s partly what makes tennis into a kind of romantic game, because it had all this history that isn’t really history, it’s legend more than actual history. Some of the ideas about how it began are quite fanciful.”

The Points

Despite its complexity, the tennis scoring system has been stable since the Victorian period.

The modern game of tennis traces back to a medieval game called jeu de paume, which began in 12th century France. It was initially played with the palm of the hand, and rackets were added by 16th century. With its strong association with pageant traditions of the French court, Wilson says, tennis was highly stylized from the beginning. Over a course of the next few centuries the game saw periods of incredible popularity, with more than 1,000 tennis courts in Paris in the 16th century. Though it’s well known for being beloved among royalty (Henry VIII was a notable and avid player, and the French revolutionaries’ tennis court oath was made on an indoor court at Versailles), it was also enjoyed among commoners and monks.

Records of scoring systems related to today’s date back almost to the beginning of the sport, but in these years the scoring was 15, 30, 45 — the math of which at least makes more sense than the modern system, as each increment was 15 points. A poem written a few years after the 1415 battle of Agincourt counts up the points — 15, 30, 45 — in a tennis game between English King Henry V and the French Dauphin. Charles d’Orléans composed a ballad around 1439 while imprisoned in an English castle after the battle of Agincourt, in which he compared life with a game of tennis and uses the French word for 45, playing on the number as both his age and the score in a tennis match. A 1505 tennis match at Windsor castle gave one player a handicap of 15. Around the 1520s Erasmus wrote a dialogue between two tennis players where one says “we’ve got 30, we’ve got 45.” Though one 16th-century English text did use 40 for a tennis score, a treatise from a few decades earlier indicated that French students were simply shortening the word “45” to “40” when they described the game (and their teacher corrected that it should be 45), writes Heiner Gillmeister, linguistics scholar at Bonn University in Germany and sports historian, who authored the thoroughly researched Cultural History of Tennis in 1998.

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But the reasons behind this counting method were obscure even then. (There’s plenty to speculate about where the English word tennis originates, too, but it definitely emerged sometime in the 15th century.) In the 1520s, for example, one Jan van den Berghe had questions: “What was not explained was how players can win fifteen points for a single stroke. It is, after all, a little curious that they count or win more than one point for a single stroke… Why is not one point given for one stroke, and two for two strokes?” Over the centuries, various theories have relied on everything from complicated multiplication to the history of scoring systems in other games to measurements of distances between lines of demarcation on early courts, but no definite answer could be found.

One of the most common suggestions, Wilson says, is that the progression is related to minutes on a clock. “It’s been suggested that the monks would look at the clock, the 4 points and somehow felt that was a good way of scoring,” she says, “and then 45 became 40, though nobody knows why.” Clocks were rapidly developing in the Middle Ages and division into a rough quarter hour was imaginable. This theory’s detractors point out that minute hands on clocks were only introduced in the late 1500s and became common even later.

The Terms

As for “love,” the word has been used since the 1700s to mean “nothing” and is also used in a variety of other games from racket sports to cards (including bridge and whist). But how it came to mean this is also unexplained.

One often repeated option traces the etymology to the French l’oeuf, meaning egg, an object the same shape as the number 0. But there is no indication the French ever used l’oeuf in relation to tennis scoring, writes American tennis player Malcolm D. Whitman in his 1932 book Tennis: Origins and Mysteries, and they didn’t write scores down, so the visual association wouldn’t cue the egg comparison. Gillmeister also writes that “love” is not how that type of loan word would be modified into English — Latin’s bovem became the French boeuf and turned into beef in English, so l’oeuf would likely have become something sounding more like leaf if that theory had held true. Gillmeister has a different loan-word idea. Perhaps it’s from the Dutch or Flemish lof, meaning honor, which would have made sense if players saw a tennis match as akin to a battle. (“Deuce” is a clearer loan word — deux is French for “two” — but the mechanism or timing of that transition is less clear.)

Or maybe it’s not a loan word at all: phrases along the lines of “neither for love nor money” had already entered the lexicon, according to Gillmeister. So the idea that a person with “love” had no money could be a plausible option as to why that might be the word for having no points in a game that was a frequent subject of wagers.

The Side Effects

By the 1800s, tennis’ popularity was in decline. The game we know as tennis today grew out of an adaptation called lawn tennis to distinguish it from the older indoor version, “real” or royal tennis. (In the U.S. that older version goes by “court tennis.”) In the 1870s, Major Walter Clopton Wingfield published rules for that new game, and a few others developed a similar game elsewhere in England at the time. Played outdoors, the court was hourglass shape and points were counted one by one. But when the All England Croquet Club set a field aside for the new game and held a championship in 1877 — the first Wimbledon Championship — they combined the new and old rules. Part of that change was a reversion to the “real” or medieval tennis scoring system: 15, 30, 40.

Lawn tennis, which was associated with the upper classes, “could afford to be more ornate, have these refinements, these quirks,” Wilson says. Even as competition increased, it remained a social spectacle. Those associations “perpetuate the retention of this weird scoring system,” says Wilson. “It becomes part of the glamour, people who watch tennis, who know tennis, can understand the scoring system, which is a bit obscure. That gives it more cachet, chutzpah, more glamour again in a funny way.”

The rules for scoring have remained almost entirely static ever since, despite some attempts to simplify it.

In 1966, for example TIME quoted James Van Alen, then president of the tennis Hall of Fame, as blaming the scoring system for the fact that “the players outnumber the spectators” in American tennis. Perhaps, the story mused, “the International Lawn Tennis Federation, which controls amateur tennis, will fall out of love with love.”

As it turns out no such luck, though one part of Van Alen’s proposed revised scoring system — tiebreakers — was added in the 1970s.

“Somewhere in this vast, great nation, there undoubtedly is a strong, agile, fiercely competitive youngster who could be the best tennis player the world has ever seen. This youngster himself may never know it. Or even care,” TIME observed in 1967. “Little that surrounds the game of tennis today is likely to appeal to him much. For a starter, there is the scoring system.”

Today, despite the scoring system, some tennis players are super stars, hundreds of thousands attend games in person, millions avidly watch televised matches and about 18 million people play the game in the U.S. alone.

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