Parallel and co-planer shafts connected by gears are called spur gears. The arrangement is called spur gearing.
Spur gears have straight teeth and are parallel to the axis of the wheel. Spur gears are the most common type of gears. The advantages of spur gears is their simplicity in design, economy of manufacture and maintenance, and absence of end thrust. They impose only radial loads on the bearings.
Spur gears are known as slow speed gears. If noise is not a serious design problem, spur gears can be used at almost any speed.
A common arrangement of spur gears is an external gear and pinion combination. If the centre distance has to be reduced, then internal gear and external pinion combination is used.