The most common gear shift lettering systems are the P-R-N-D, the L-H, and the M system. The P-R-N-D system is useful on most automatic transmission vehicles and consists of four gears: park, reverse, neutral, and drive. The L-H system (Low-High) is used on certain types of vehicles, such as off-road vehicles and large trucks. The M system (Manual) is used on vehicles with a manual transmission and consists of six to eight gears.