prismatic joint

A prismatic joint is a joint that lets one link slide straight along a single line, the way a drawer pulls in and out of a desk. It cannot rotate or wobble — it only travels back and forth along one direction. Like a revolute joint, it allows exactly one independent motion, so it too has one degree of freedom, but here the single number that describes its state is a distance (how far it has extended) rather than an angle.

If revolute joints are the hinges of robotics, prismatic joints are the telescoping rails and pistons. You see them anywhere a robot needs to push something out, raise a platform, or extend a reach in a perfectly straight line — a 3D printer's bed gliding forward, a forklift mast lifting, a CNC cutter tracking along its rail. They are valued precisely because straight-line motion is simple to control and predict; the part goes exactly where the slide points it.

Engineers label a prismatic joint with the letter P. Robots often mix the two types: a SCARA arm, for instance, uses revolute joints to position the hand over a spot and then a prismatic joint to plunge straight down onto it. By combining R and P joints, designers get both the wide sweep of rotation and the clean, direct push of sliding.