In some countries there is a fixed charge for personal services. A certain percentage may be added to the bill at a hotel or restaurant "for the service". In other places the customer may be expected to give a tip, or a small amount of money, as a sign of appreciation whenever services are performed. In the United States there is no standard practice in regard to tipping . The custom is more common in a large city than in a small town. A native American may often be in doubt about when and how much to tip when he is in a city that is strange to him. In general, however, a tip is expected by the porter who carries your baggage, by taxi drivers(except, perhaps, in a small town), and by those who serve you in hotel and restaurants.
When you pick up your incoming baggage at an airport, you may tip the man who takes it to the taxi or airport bus. He usually expects one dollar a bag for his service. In some cities the taxi that takes you to your hotel may have one meter (计价器)that shows the cost of the trip and another that shows a fixed charge, usually about 50 cents, for "extra". In some cities the taxi driver may expect a tip in addition to the "extra", especially if he carries your suitcase. If no "extra" is charged, a tip is usually given. Hotels generally do not make a service charge, though there are places where one is added. It is customary (惯常的), however, to give something to the porter who carries your suitcases and shows you to your room. In case of doubt, one dollar for each bag he carries is satisfactory. In a restaurant you generally leave about 15 percent of the bill in small change on that table as a tip for the person who has served you. A service charge is generally not included in the bill except in some of the larger, more expensive places. If the order is small----a cup of coffee at a lunch counter, or something of the sort----a tip is not usually expected. According to the passage, the practice of giving tips ______.