In the U.S., tips - this is legitimized form of additional pay in the service sector. Gratuity is not included automatically, but left to the guest. It is distributed in a taxi, airports, hotels, restaurants. Porters underpaid 0,25 -0,5 dollars for one seat. A few more buttons gets ( "bellboy") in a hotel (0,5 - $ 1 per seat). Tipping is accepted to give a head waiter, porter, maid. Tipping a waiter, taxi driver are 10-15% of the amount of the account. In New York, this amount can reach 20%. You should never offer money police officer or official. This attempt may be considered a criminal offense.
