The first mechanical watches manufactured by Rolex and other Swiss watchmakers in the late nineteenth century and early twentieth century required one to wind the crown to power the movement inside the case. The Perpetual rotor, developed by Rolex in 1931, changed that. With the addition of a half-moon shaped oscillating weight to rotate 360 degrees with the motion of the wearer's wrist, the Perpetual rotor provided enough energy for the movement to power itself. 

You may have noticed that, save the models in their Cellini line, Rolex watches have the words 'Oyster Perpetual' printed on the dial. The word Oyster refers to the waterproof Oyster case, developed by the watchmaker in 1926. The Perpetual part refers to the rotor and indicates that the timepiece is in fact self-winding or automatic. There are still luxury watchmakers, like Panerai and others, that still offer mechanical watches that require winding, but all of Rolex's current models are self-winding.

For more information on Rolex's Perpetual rotor, check out my previous post about the innovation in the Tech section of this website. You can also find information on Rolex's official website at rolex.com.