Most drinking establishments of the style that I prefer have a bell mounted on the wall behind the bar. When I was bartending in Montréal, ringing the bell was the signal that you were buying the house a round, or that a customer had made that offer. (In a multiple bar complex like Cheers or Woody’s, it would just apply to those seated at that one bar.)
I was having dinner at Rosy Tomorrows in Danbury CT on Monday with a friend and colleague of mine who used to own a pub. I spotted the bell over Rosy’s bar and asked my friend if he used to have one behind his bar. He said yes, and that the reason it was rung at his establishment was to celebrate a big tip. After a while it became apparent that a large gratuity was the trigger event for the bell to be rung at Rosy’s as well.
I’m fully behind this latter practice. You are fragrantly thanking the generous patron, and encouraging similar behaviour in others.
By the way, when I went looking for an image of a bell for this post, I found this one at an online bar supplies vendor in the UK. They call it a “Last Orders Brass Bell,” so there is apparently at least one other use for them in bars, too.