A gratuity (also called a tip) is a sum of money customarily tendered, in addition to the basic price, to certain service sector workers for a service performed or anticipated. Tips and their amount are a matter of social custom, and the custom varies between countries and settings. In some locations tipping is discouraged and considered insulting; while in some other locations tipping is expected from customers. The customary amount of a tip can be one of a specific range of monetary amounts or a certain percentage of the bill. In some circumstances, such as with U.S. government workers or more widely with police officers, receiving gratuities (or even offering them) is illegal, as they may be regarded as bribery. A service charge is sometimes added to bills in restaurants and similar establishments. Tipping may not be expected when a fee is explicitly charged for the service.