FIA regulations state that drivers may have no more than three cars available for use at any one time. Usually a team will bring three or four cars to a race; a race car for each of its two drivers, and one or two spare cars for use by either driver. Use of those spare cars is subject to various restrictions.
If a driver switches car between qualifying and the race then he must start the race from the pit lane. A change of car is not allowed once the race has started.
There are also restrictions on engine use. Each driver may use no more than one engine for two consecutive Grand Prix meetings. If an engine change is required ahead of qualifying at either meeting, the driver will drop ten places on the grid for that event. If the change is made after qualifying, the driver goes to the back of the grid.
For the purposes of this engine regulation, the Grand Prix meeting comprises Saturday's practice and qualifying sessions, plus Sunday's race. During Friday's practice sessions, drivers may use alternative engines, with no penalty should such an engine fail.
If a driver fails to finish a race due to reasons beyond his or his teams control, he may start the next meeting with a new engine without incurring a penalty.