Leonardo da Vinci sketched a ball bearing as an early way to reduce friction between moving parts.
His design used rolling balls placed between surfaces, allowing one component to move more easily against another. Rolling contact generally creates less friction than sliding contact, which is why the basic principle remains important in modern machines.
Leonardo studied bearings while developing mechanical systems and machines. His notes show an understanding that the balls needed to remain separated; without spacing, they could bunch together and interfere with motion.
Leonardo was not the inventor of every later bearing technology, and his sketch was not a modern standardized bearing. The historical importance lies in the clear early treatment of the rolling-element principle, centuries before industrial manufacturing made ball bearings common.