The bicycle chain couples the chain wheel that is driven by the pedals to the rear wheel that propels the bike. The diagrams below compare a design in a manuscript of Leonardo da Vinci to a modern bicycle chain. Engineering is well founded in history!

From: Leonardo da Vinci, Codex Atlanticus (circa 1490)

Sloane, "Sloane's Complete Book of Bicycling,"
Simon & Schuster (1995)