In Belt drives when the belt passes from the slack side to the tight side a certain portion of the belt extends. Similarly when the belt passes from the tight side to slack side it contracts. Hence creep in belt drive is due to uneven extensions and contractions of the belt when it passes from tight side to slack side.
This phenomenon of alternative stretching and contraction of the belt results in relative motion between the belt and the pulley surface. This relative motion is called creep.

When two parallel and coplaner shafts are connected by gears having teeth parallel to the axis of the shaft called spur gears, the arrangement is known as spur gearing.

Another name given to the spur gearing is helical gearing in which the teeth are inclined to the axis of the shaft. There are single and double helical gears. The double helical gears are known as herringbone gears.