The stopper comes off. The bartender tips the bottle toward the glass, tilting her head slightly as she watches wine fill the glass. Her customer might watch, too, wondering when the pour will stop — and if they’ve paid $12, $14 or more, maybe silently grousing if they feel shortchanged.

It’s a familiar tango. As more restaurant diners, especially solo ones, bypass bottles of wine for single glasses, by-the-glass lists have grown, as has the standard wine pour — from 4 or 5 to 6 ounces, one of the most generous pours in the world. (“Un verre du vin” in France, for instance, is still about 4 ounces, and even less in Iceland — but much more in Hungary).

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Not so long ago, I watched a bartender second-guess himself after he poured my glass of Nero d’Avola, then splash more in. Are regulars (or even big spenders) sometimes gifted with more than 6 ounces? I asked Paulo Villela, corporate beverage director for Bohlsen Restaurant Group, the company behind Huntington’s Harbor Club at Prime, Verace in Islip, and a handful of other Long Island restaurants. “We try to avoid that,” said Villela. So staff can know exactly how much they’ve served, all Bohlsen restaurants stock the same stemware: hefty 18-ounce glasses etched with strategically placed logos so that pourers know where to stop, every time. Such a glass also allows the wine to aerate. “We don’t believe in those little glasses,” Villela said.

At East Setauket’s Madiran The Wine Bar, owner and sommelier Jacqueline Malenda also uses glasses etched with a faint diamond at the 3-, 5-, and 6-ounce marks, the last of which is Madiran’s standard by-the-glass pour. “Since the glasses are normal size, the wine is not dwarfed in the glass,” said Malenda. “I’ve worked elsewhere where other shapes or sizes of glasses were used, and found that customers were confused regarding the fact that the pour was the same, but appeared differently.”

So whether it’s a balloon glass into which your wine seems to disappear, or a daintier glass filled almost to the brim, chances are the bartender knows exactly how much they’ve pushed across the bar.