Parallel motion is a mechanical linkage that was first invented by James Watt in 1784. It was developed for use on his double-acting Watt steam engine and replaced the previous Newcomen beam and chain setup.
His new engine allowed power to be harnessed in both the upward and downward strokes of a piston effectively doubling the efficiency. It was termed "parallel motion" by Watt because both the piston and the pump rod were required to move vertically, parallel to one another.
It would prove immensely successful and became a critical innovation that helped define mechanics today.