A gear is a toothed wheel that passes motion and power along. Gears are one of the most important machine parts used by people to make things move. Watches and clocks use gears to move their hands. Automobiles, trains, and planes use gears to turn the power of their engines into the motion of their wheels or propellers. Even wind-up toys use gears.

Toothed wheels, or gears, have been known and used for over 2,000 years. A water clock with gear wheels was built by Ctesibius (1st century B.C), a Greek mathematician living in Alexandria, Egypt. To build a clock required a fairly advanced knowledge of gears.

The first known geared mill was built about 27 B.C. It was probably the first mill to use machine elements like gears. Around the beginning of the Christian era gears were a part of many new machines, such as clocks and various kinds of waterwheels and windmills, that were beginning to be a part of daily life. Roman columns built just after the time of Christ were decoratively sculptured with gears, indicating an awareness of their importance.

Later, Leonardo da Vinci (1452-1519) suggested broader use of geared machines. His ideas were not followed until the next century.

For centuries gears were made of wood or wood with stone teeth set in. occasionally a metal like iron, tin, or bronze was used. But it was not until precision gear-cutting machines were invented in 1855 that gears came into wide use. Since then many types of gears have been developed. Accurate gears were very important if friction was to be reduced and power was not to be lost in the transfer.

The first metal gears were cast (formed from a mold). Now, when very accurate gears are needed, gears are cut out from metal disks instead of being molded. Cast gears are still used in inexpensive machines and appliances or when gears are used only once in a while. Gears for watches and clocks are stamped from thin sheets of metal.