Have you ever seen someone at a bar put a coaster on top of their drink and then proceed to walk away? If you’ve ever questioned this practice — Where are they going? Why would they ever leave their drink? — you’re likely not the only one. Bar culture has its own unspoken language, different quirks that seem strange to a passerby, but to the experienced barfly, the signal is clear.

To help decipher this, VinePair spoke with Bryce Kephart, bartender at the Horse Inn in Lancaster, Pa., a tavern known for its shape-shifting abilities as both a local watering hole and an inspired cocktail bar. “When I see someone put a coaster on their drink, my reaction depends on a couple of things: Are they finished with the drink? Is it still half full? Do they still have a tab open?” says Kephart. “But most of the time it just means they are leaving their drink and they will be back.”

For many, this is a universal sign, arguably from the days of yore, to say you’re going to finish this beer, or that spot is already taken. Kephart notes that it is usually something he sees from an older crowd, “In older men it seems to be more narrowed in,” says Kephart. “In my experience, older guys tend to wander more around the bar.”

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While coasters are preferred, Kephart says a bar napkin can be used as an easy fix. “In the past, I have had to quickly throw napkins on people’s drinks when they have left,” says Kephart. “In that case, it is more of a signal to other people.”

This rule does have its limits, though. While leaving your drink unattended is not advised, and could even be considered dangerous in some settings, if you do need to step away, try to keep your excursion as short as possible. This might be feasible to do on a Monday, but be sure to read the room on more bustling evenings, as the courtesy of other patrons may not prevail on a Friday or Saturday night. And so the bartender, who wants to be serving as much as they can as effectively as possible, isn’t beholden to your drink. “I have had other people sit down, pushing the beer to the front to say, ‘Clear this,’” says Kephart. “People are definitely seat hungry on the weekend. They will forget or ignore obvious things and cues to get a seat.”