Zach Helfand, writing at The New Yorker:
English coffeehouses were said to set out urns inscribed with “To Insure Promptitude.” Customers tossed in coins. Eventually, the inscription was shortened to “tip.”
I didn’t know I needed a ~4,700 word article on the history and subtleties of tipping, but this was a good read. We’ve recently been having windows and doors installed in our house, so the vagaries of how much to tip have been freshly on my mind.