Spur gears :

Spur gears are the simplest and most common type of gear. Their general form is a cylinder or disk. The teeth project radially, and with these "straight-cut gears", the leading edges of the teeth are aligned parallel to the axis of rotation. These gears can only mesh correctly if they are fitted to parallel axles

A pair of spur gears for mounting on parallel shafts. The 10 teeth of the smaller pinion and the 20 teeth of the wheel lie parallel to the shaft axes