Human Power and Propulsion

The diagrams below show the effect of correct chain length for a deraileur gear. The rear wheel's position is adjusted until the pulley cage on the deraileur swings between the two positions show. The left hand diagram shows the cage position when the chain is on the smallest freewheel cog and the smallest chainwheel. The other diagram shows the same chain riding on the largest cog and the largest chainwheel. Links may be added or removed from the chain until these two conditions are reached.

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