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Why do people put coasters on top of their drinks at bars?

Bars are incredibly busy places, and bartenders and servers are taught early on to remove abandoned drinks from a bar or table as quickly as possible. A clean bar is a good bar, after all; besides, they might need to wash and reuse that glassware.

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If you are drinking alone and need to head to the bathroom (because bringing it in with you is disgusting) or step outside, putting a coaster on top of your glass lets bartenders know you’re coming back. Most bartenders will know the signal and won’t pour out your drink, unless of course you’ve stepped out for an hour-long conference call. There are limits, after all.

A coaster on top of the drink is also a signal to fellow drinkers when the bar is crowded that the seat you were sitting in is occupied. This is only appropriate to do if you plan to be away from the seat for only a few minutes, maybe to run to the bathroom or go grab a friend who’s just arrived and is having a hard time finding you. It is an absolute no-no to hold the seat if you plan to step outside for that conference call I mentioned earlier. You don’t get to claim a seat all night if you are outside making deals or breaking up with your significant other.