“Actually, we have Starry,” the counter clerk said. It was early spring of this year, and I was ordering a lemon-lime soft drink. I had asked for Sprite but was told that the establishment, a Pepsi shop, had Sierra Mist instead. But wait, it didn’t have that either, because Pepsi had just killed off its 22-year-old lemon-lime brand and replaced it with a new one: Starry. Did I want a Starry? I guessed so. What was the difference? I couldn’t tell; it tasted like lemon-lime soda, a flavor too ordinary to remark upon.

But wait: Why should “lemon-lime,” as a combination, be so ordinary? Lemons and limes are both tart citrus, but very few other foods or beverages, packaged or prepared, put the two together. Let’s face it: It’s not normal to squeeze both lemons and limes onto your tacos, or bake them into key-lime-and-lemon pies. And do Sprite or 7up or Sierra Mist or Starry really even taste like the merger of these fruits? Or is “lymon,” as Coca-Cola has sometimes characterized Sprite’s flavor, merely a marketing gimmick that has duped me, you, and the world?

Seeking answers, I embarked on a voyage into the history of citrus and soda—and learned from my travels that lemon-lime is less a flavor than an archetype, a bright, vibrant antidote to heat and sluggishness that gets delivered through the excited medium of carbonation. Indeed, a certain spectral lemon-lime-ness has been with us since the very start of manufactured fizzy water in the 18th century. It’s the ghost, you might say, in the soda machine.

By the fall of 1929, Charles Leiper Grigg had failed at making soft drinks once already. His orange soda, Howdy, had been crushed by Orange Crush. 7up would be his follow-up and great success. From the start, it was flavored lemon-lime—a combination Grigg would come to dominate. In that sense, Sprite, Sierra Mist, and Starry are his children too.

The time was right for innovation. In the 1920s, improved bottling technology, reliable caps, and modern refrigeration made soda a viable packaged good for the first time. Aspiring soft-drink designers of the Jazz Age saw dollars in bubbles and tripped over one another to establish the next big thing. Therapeutic properties were a natural selling point: Fizzy mineral waters had long been considered curative on their own terms—mineral springs were the original spas—and corner shops fused them with medicinals. That’s why John Pemberton put the stimulants cocaine and kola nut in his concoction Coca-Cola, first served in 1886. Pepsi was so named because it was thought to provide relief for dyspepsia. And the mood-enhancing lithium that Grigg put into his drink—which was originally called Bib-Label Lithiated Lemon-Lime Soda—followed this tradition.

But the other half of his concoction—the lemon-lime—carried on a tradition too. The essence of that flavor, its sharp brightness on the tongue, goes back at least to 1772, when the English chemist Joseph Priestley published his Directions for Impregnating Water With Fixed Air, the first influential manual for the artificial manufacture of carbonated water. Inheriting from the perceived health benefits of natural spa water, Priestley hoped to win over the British Admiralty on a method for improving water drunk at sea by, well, impregnating it with fixed air. Even without added flavors, carbonated water contains carbonic acid, giving it a mild taste in addition to its fizz: Plain synthetic seltzer has the sensibility of citrus.

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In his treatise on artificial carbonation, Priestley makes the case for soda as a way of treating or preventing scurvy, partly on the grounds that it opposes “putrefaction” of the humors caused by dampness, poor discipline, and a noxious diet (among other factors). People were starting to recognize that citrus fruit could serve that purpose too, but according to Ken Albala, a food historian and professor at the University of the Pacific, nobody made the scientific connection among citrus, vitamin C, and bright, acidic flavors for another 100 years or so.

During that century, soda’s stock would rise. Pharmacists and fountain-soda jerks mixed concoctions of fizzy waters and flavorings, both for gustatory delight and as cures to common ailments. Lemonades were popular, along with flavors from roots, spices, and fruits. Lemon and lime were combined into soda water at some of these pharmacies, and then again in many locally distributed bottled sodas that were already on the market when Grigg hit it big with 7up.

But even then, why? “No idea!” Tristan Donovan, the author of Fizz: How Soda Shook Up the World, told me. The beverage flavor pairing “just seems to have been around.” As commonplace as it seems today, lemon-lime is, or was, historically aberrant. Lemon and lime are related, both sour, small-bodied citrus fruits with tennis-ball-colored rinds. But in the long history of human gastronomy, the two rarely appear together. Try to name a classic dish that uses both lemons and limes? Albala couldn’t think of one. Likewise, lemon and lime are common mixers and garnishes for cocktails, but generally not together—a gimlet is a lime drink; a Tom Collins is a lemon one.

Ali Bouzari, a culinary scientist and packaged-food consultant, pointed out that some Iranian food, such as chicken for shish kebab, might be marinated in onion, saffron, salt, lemon, and lime. He also thought of Peruvian cuisine, in which preparations for ceviche, for example, might deploy both fruits. But Bouzari admitted that these exceptions may only prove the rule: Lemon and lime aren’t common culinary companions, even in the cuisines of regions where both fruits grow.

At least that’s the case for natural lemon and lime. Artificial fruit flavorings, Bouzari told me, have been popular since Grigg’s time and before—and lime, in particular, could have been easier to find back then as a flavor than a fruit. According to Judith Levin, the author of Soda and Fizzy Drinks: A Global History, some early “lime” sodas were really just lemon ones, so named “because lime was fancier.” Where lime flavoring was really used, the product might have been disgusting. Even today, a lemon taste is easier to produce in syrups or extracts. “Out of every hundred lime samples I smell,” Bouzari told me, “at least 80 smell like cleaning product.” He referred to these as “weird Franken-lime” flavors, and said he found them to be “aggressively funky.” A discerning soda flavorist seeking novelty might have added lemon to hide the sweaty taste of fake lime, he said, though he acknowledged the idea was complete conjecture.

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Perhaps Grigg’s choice of flavor was an afterthought. His product’s major hook might have been lithium, a psychoactive drug. (7up “takes the ‘ouch’ out of grouch,” one of the soda’s early slogans said.) Is it possible that Grigg, still citrus-addled by his experience with Howdy, had simply chosen lemon and lime to double the alliteration, Lithiated Lemon-Lime?

It’s impossible to know. I spoke with Rachael Nadeau Johnson, the collections manager at the Dr. Pepper Museum in Waco, Texas—which also houses a collection of materials related to the parent company’s other brands, including Squirt, A&W, and 7up. She said some lines of evidence suggest that Grigg threw in lithium at the last minute, but “there’s not a lot of data about the early years.”

Whatever the case, Grigg’s timing surely helped. The wholesome lemons that go into lemonade met the tart limes from a still-illegal gin rickey, and lithium made everyone feel good. America would need it: Shortly after Grigg’s launch, the stock market crashed and the country fell into the Great Depression. A little bright acidity in the form of a cold bottle of 7up felt just fine, just fine.

Feeling fine is the product soda sellers sell. “Enjoy Thirst,” “It’s the Real Thing,” “You’re in the Pepsi Generation,” “The Pause That Refreshes”: For a sugar-sweetened soda to succeed, it has to taste good, but it also has to persuade consumers to choose it over any of the other roughly equivalent options available. Product design is important, but branding and marketing are essential. “It all works together,” Gary Hemphill, the managing director of research at the Beverage Marketing Corporation, told me. “You have to hit on all cylinders.” (All the beverage brands I contacted for this story, including PepsiCo, Coca-Cola, Keurig Dr. Pepper, and National Beverage Corp., either didn’t respond or declined to speak with me.)

Once 7up appeared in 1929, the lemon-lime category quickly became entrenched. The drink was extremely successful during the Depression, Nadeau Johnson said, perhaps as an alcohol alternative and perhaps as a mixer for bootleg booze. After Prohibition ended, 7up-cocktail recipe books proliferated. (You don’t have to mix your drink, encouraged one such recipe that Nadeau Johnson described to me, because 7up’s bubbles mix it themselves.) But the brand faltered, and seemed old-fashioned by the late 1950s; sensing opportunity, the Pepsi-Cola Company came out with Teem and the Coca-Cola Company launched Sprite. 7up fired back with a counterculture-themed ad campaign and the slogan sneer “The Uncola.” Pepsi’s Slice showed up in 1984, and then Sierra Mist in 2000, but neither developed the strong sense of self of 7up or Sprite. Starry is too new to judge, but PepsiCo seems to be framing the product with a brightly colored, Gen Z–optimism appeal, perhaps in an attempt to recapture a new Pepsi Generation lost to gently flavored seltzers.

It made me wonder: Why should any of these brands succeed? What makes someone choose a lemon-lime variety of soda, no matter what it’s called, instead of a cola or a root beer or an orange pop? Hemphill didn’t know—variety, maybe, or demographics, or regional preferences, or who knows what else.

Everyone I spoke with seemed to think that lemon-lime soda is associated with summer, or with relief from summer heat. All soda brands are billed as “refreshing,” a feature Albala connects with the acidity intrinsic to carbonated soft drinks. It’s in “the way something goes across, scours your palate,” as he put it. Lemon and lime flavors provide the same effect of their own accord. Rich sauces, or hot and spicy flavors, generally get balanced out with sour tastes, often drawn from lemon and lime. Like cucumbers, these fruits are cool.

Despite its seemingly descriptive name, lemon-lime’s taste is harder to describe. Sampling drinks in the process of writing this story, I wasn’t sure that lemon-lime soda tastes that much like lemon or lime. Just crisp, cool, thin, lithe. Why? Bouzari told me about a phenomenon called mixture suppression. The more things our brains consider at once, the more they blur together. In some cases, that produces confusion (a muddled taste), but in others it can be clarifying: “Tropical” isn’t mango or guava or piña colada—it’s just tropical. The harsher, not-quite-orange flavor of Mountain Dew, which insiders call “heavy citrus,” offers another example. So does mixed berry. It isn’t blueberry or strawberry or raspberry or even all of them together, but a sort of tangy-dark amalgam. You know, mixed berry.

“Lemon-lime is more of a vibe than a flavor profile,” Bouzari concluded. It is a gesture or a suggestion or a concept or an idea. Once you understand lemon-lime this way, as a category of affective response rather than a flavor, a band of soda atmospheres emerges from the mist. On one end, Bouzari proposed, is our hero lemon-lime, which concretizes the crispness—the “acidulous taste” that Priestley identified in fizzy water itself. On the other, its opposite: the dense, dark complexity of cola—earthy, spicy, warm. It too is not a taste but a sensibility—“Coke Is It!” In the middle, fixed flavors (root beer, ginger ale) find their place, along with wild cards (Dr. Brown’s Cel-Ray, Mountain Dew). But across the spectrum, many sodas are imbued with citrus, linking them to lemon-lime. While brand-name soda formulations are closely guarded secrets, colas are generally known to be produced with citrus oils, along with spices and vanilla. Similar flavors can be discerned in the cola-adjacent Dr. Pepper. (Dr. Pepper, by the way, is a “pepper” drink. I wish I had time to explain further.)

Americans have been on the outs with soda for some time—Hemphill says overall consumption is down 21 percent since 2004. But soda still makes up a large proportion of the beverage market; as such, it offers one of the most widespread avenues for developing shared tastes. Tastes in the sense of flavors, to be sure, but also as penchants of a broader sort, such as the appeal of being a Sprite guy or a 7up gal. “Marketing is all in your head,” Bouzari said, “but that’s where all flavor lives anyway.”

All of this has been true for a century or more, but the details fade into the familiarity of pop life. The strange, sudden fact of Starry, Pepsi’s new lemon-lime soda, helped me see soda’s deeper truth. Lemon-lime soda is not a product to be innovated, but merely iterated. It’s a jazz standard, Bouzari told me. Every beverage company must have one in its suite of products, and you, the drinker, must be able to recognize it. Not just for its taste, but as a concept that helps you identify which drink might be right for you at a particular moment. Lemon-lime plays a special role in that unconscious calculus, because all sodas are acidic, and because artificial carbonation itself has always been haunted by the ghosts of lemons and limes.