Have you noticed that at most restaurants, etc., when you pay using a credit card an automatic 15% gratuity is added to your bill? Then, when the credit card slip is brought back to you to be signed, there is a line for a tip on the amount that already includes the gratuity. If you then tip 15%, you tipped over 32%.
When I have queried this with managers, I usually get the answer that it is the banks that design the credit card slips. One even argued that there was a difference between a gratuity and a tip, because a gratuity is compulsory and a tip is voluntary. In my dictionary, a tip can be defined as a gratuity and a gratuity as a tip.
Now, having paid a 15% gratuity on my bill, I write “Already included” on my credit card slip in the space for a tip.