At controlled airports, the airport surface area is split into the movement and
non-movement areas. The movement area is the control tower's area of
jurisdiction, so a taxi clearance is required prior to entering the movement
area. In the non-movement area, however, you may move the airplane without
clearance or communications with the control tower.
A clearance must be obtained prior to taxiing on a runway, taking off, or
landing while the tower is in operation.
A clearance must be obtained prior to crossing any runway. The controller
will issue an explicit clearance for all runway crossings.
For example, the ground controller might say, "Cessna 66769, runway one niner,
taxi via taxiway alpha, hold short of taxiway golf".
The pilot must read back to the controller all hold short instructions, the
runway assignment, and any clearance to enter or cross a runway.
Always ask for clarification if there is any aspect of your clearance of which
you are not 100% certain. If you are unfamiliar with the airport and want
to make sure you aren't going to make a wrong turn somewhere, you can ask for a
"progressive taxi". When a pilot requests progressive taxi, the controller
provides step-by-step taxi instructions. Progressive taxi may also be
useful at an airport where extensive or confusing construction is taking place.