If the axes of the first and last wheels of a gear train are co-axial, it is called _______. 

Concept:

Reverted gear train

• When the axes of the first gear (i.e. first driver) and the last gear (i.e. last driven or follower) are co-axial, then the gear train is known as a reverted gear train.
• The motion of the first gear and the last gear is like.
• The reverted gear trains are used in automotive transmissions, lathe back gears, industrial speed reducers, and clocks.

Simple gear train:

• When every gear axes are fixed relative to the fixed frame and, there is only one gear on each shaft.
• The number of shafts may be any.

Compound gear train

• When any shaft has more than one gear (first shaft or second shaft or intermediate shaft) i.e. two gear have the same angular velocity, the gear drive is called compound gear drive. 

Epicyclic gear train

• When the gear train is having a relative motion of axes it is called the epicyclic gear train.
• The axis of at least one of the gears also moves relative to the frame.