Gratuity, the tip

tip
'tip' is the word for gratuity in the sense of your appreciation for a prompt service. tip means: to insure promptness.
In former times people were used to give the tip before receiving any service, but later peolple thought it would make more sense to wait until the service was performed - so the tip could be adjusted to the level of satisfaction.
So far the history - what is now the appropriate tipping amount?
In restaurants it's usual to give 15% tip on the total amount. Note: the amount is not depending on the quality of the food but on the friendliness and ability of the waiter/waitress.
Service personnel lives on 'tip', it's important for them and in the US, this gratuity is not included in the bill (called check in the US). Exception: you will often see that tip will be automatically added to the total amount of your bill when parties of more than 6 or 8 persons sit together in a restaurant.
Tipping in hotels:
bell man / doorman etc.: 1$ per piece of luggage or when getting you a taxi, room maids approx. 1-2$+ per night, depending on the level of hotel.
Taxi driver: 10-15%, min. $1
At the airport: porters get usually 1$ per piece of luggage. You may give more, if you have heavy luggage.
Valet parking / hotel parking / restaurant parking: 1$-$2 for getting your car.