A gear used when the rotation axes of two gears are not parallel.
Since the tooth profile changes depending on the inclination of the shaft and the corresponding gear, the two gears that mesh are designed as a set.
Compared to a pin gear that can perform the same role, it is more difficult to produce, but it can transmit rotation more accurately with less backlash.
Spur gears can be used if the rotation axis is inclined at 5 degrees or 10 degrees, but if it is 60 degrees or 90 degrees, it is effective to use bevel gears or pin gears.