Leonardo da Vinci sketched a ball bearing around 1498 to reduce friction between moving parts.
His design used loose balls so that two surfaces could move with rolling contact rather than sliding directly against each other. Rolling contact generally produces less friction, making motion smoother and reducing wear. The drawing is often cited as an early step in the development of the modern bearing.
Leonardo did not invent every later feature of industrial ball bearings. Modern bearings use carefully manufactured rings, hardened balls, cages, lubrication, and precise tolerances. His Renaissance sketch was a conceptual design rather than a mass-produced machine component.
A common mix-up is calling the drawing a complete modern bearing. It is more accurate to say that Leonardo documented an early ball-bearing concept. The idea later became important in clocks, vehicles, turbines, electric motors, and many other machines.