Spur gear  was used began ancient times. Modern gears are a refinement of the wheel and axle. Gear wheels have projections called teeth that are designed to intersect the teeth of another. When the teeth fit together or interlock in this manner they are said to be in mesh. Those in mesh are capable of transmitting force and motion alternately from one to another.

The gear transmitting the force or motion is called the drive gear and the one connected to it is called the driven one.Gear products are used to control power transmission  in these ways. One is changing the direction through which power is transmitted (i.e. parallel, right angles, rotating, linear etc.). Another one is changing the amount of force or torque. The last one is changing RPM. In order to design, build and discuss gear drive systems, it is necessary to understand the terminology and concepts associated with gear systems.

The following are some important terms that need to be known.The first and the most important are the spur gears which are cogged wheels whose cogs or teeth project radially and stand parallel to the axis. And the Diametral Pitch (DP) describes the tooth size. It is expressed as the number of teeth per inch of Pitch Diameter. The teeth size varies inversely with Diametral Pitch.Addendum (A) refers to the distance from the top of the tooth to the Pitch circle Dedendum refers to the distance from the Pitch circle to the root circle. Clearance (C) refers to the radial distance between the top and bottom of gears in mesh. Rotation is also an important term in the use of gear products. Spur gears in a two-gear drive system will rotate in opposite directions. When an intermediary gear set or idler gear is introduced between the two gears the drive gear and the last gear will rotate in the same direction. Becides, there are still many others such as ratios, velocity, and backlash, etc. All of them are necessary for the clear understanding of the bevel gear drive systems.