If a goblet filled with seven fingers of Merlot is sloshed your way across an assumedly sticky bar, it might seem like the bartender has taken a liking to you. That may be true (you’re so great!), but, sadly, one thing is certain: The person pouring the wine doesn’t care about what’s in the glass.

“Air is a friend to wine when you’re trying to drink it,” Cedric Nicaise, wine director of Eleven Madison Park, the world’s top-ranked restaurant, says. “Whereas it’s the enemy of wine when you’re trying to store it.”

When you pour something into a glass, only the surface of the liquid comes into contact with air. Think about cooking a burger on a grill. The side directly against the grates gets a nice dark char, while the other half stays pretty raw. When you fill a glass with wine, only the top part hits air, while the rest stays “raw.”

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And while aeration doesn’t exactly “cook” the wine, it does help open up its flavors and aromas. That’s why people swirl wine in a glass when they taste it, or use decanters so juice that’s been sealed up in a bottle can “breathe.”

The appropriate serving size for wine by the glass in a bar or restaurant is 5 ounces, Nicaise says. Modern wine glasses hold anywhere from 15 to 26 ounces of liquid, providing ample real estate for energetic swirling.

“You want some space between the top of the glass and the wine,” Nicaise says, “That way you can swirl and expose some of the wine to oxygen.” If your glass is filled to the brim, you can’t swirl. You can barely lift it off the bar or table without spilling.

One way to guarantee your wine receives adequate aeration is to ask for your portion to be split between two glasses. This might seem fussy, but a good wine bar will happily comply.

Of course, you can also just drink your half-jug of Merlot and move on with your life.

“It all depends on what you’re going for in terms of your consumption,” Nicaise says. “If you’re trying to consume a high quality of something, you want the right size pour. If you just want a high quantity of something, then a large pour will be right for you.” Diplomacy, like wine, is an art.