What are people talking about when they say a wine has "legs"?

Hello there! I'm Dr. Vinifera, but you can call me Vinny. Ask me your toughest wine questions, from the fine points of etiquette to the science of winemaking. And don't worry, I'm no wine snob—you can also ask me those "dumb questions" you're too embarrased to ask your wine geek friends! I hope you find my answers educational, empowering and even amusing. And don't forget to check out my most asked questions and my full archives for all my Q&A classics.
Dear Dr. Vinny,
What does it mean when a wine is swirled in a glass and is said to have “legs?”
—Connie, United States
Dear Connie,
I was just having lunch with a winemaker (yes, cartoon characters need to eat too!) and we were lamenting that so many primers on how to appreciate wine still mention these translucent streaks that cling to the sides of a glass after you swirl it. It feels like a confusing point to linger on, because the appearance of a wine's “tears” or “legs” doesn't really mean anything in terms of a wine's quality. They are caused by the evaporation of alcohol after swirling, which affects the surface tension of the wine clinging to the glass. In theory, higher alcohol wines might display more streaks, and the viscosity of sweeter wines can slow the streaks down. It definitely has nothing to do with a wine’s quality.
I can’t tell anything by looking at them. All wine leaves legs, but it’s not like you can look at a glass and go, “Ah, this wine has an alcohol by volume of 14.1 percent!” If you want to know the wine’s alcohol percentage, check out its label.
Commenting on a wine’s legs is kind of like saying a wine is wet. I think “legs” are a throwback to how wine used to be discussed, when it felt like there was a whole mythology and secret language to how a wine should be tasted and appreciated. I’m glad wine discussions are more straightforward these days—no more secret handshakes required.
—Dr. Vinny