Driving a truck is a professional job that includes much risk and expertise, and to be a legal truck driver, a driver must have a license to drive a truck. This license is called a Commercial Driver's License. To obtain it, a person must pass a number of written exams. These include a general knowledge test, and air brakes test and the combination vehicle test. Upon passing these tests, a driver is given a 6-month permit, during which time they must learn how to drive a semi. The driving test for proving this includes showing a driver is able to park, back up, perform a vehicle inspection before driving, and are able to conduct an air brake test. If a driver passes this, they get their truck license. Many truck drivers choose to continue their training and take optional tests for hauling hazardous materials, tankers and double and triple trailers. These extra qualifications lead to a higher level of license and a higher pay rate for truck drivers.