The height from the pitch circle of a gear to the tip of the tooth is called as Addendum

The depth of the tooth space of a gear below the pitch circle is called as Dedendum

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. In Herringbone gearing the axial thrust is minimized.

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.