When the motion between the elements, forming a pair,is such that the constrained motion is not completed by itself, but by some other means, then the motion is said to be successfully constrained motion.

Sliding Pair: When the two elements of a pair are connected in such a way that one can only slide relative to the other, the pair is known as a sliding pair. The piston and cylinder, cross-head and guides of a reciprocating steam engine, ram and its guides in shaper, tailstock on the lathe bed etc. are examples of a sliding pair. A little consideration will show, that a sliding pair has a completely constrained motion.