Angular Contact Bearings, also called duplex bearings, are machined as matched sets.  The pairs are furnished with the race faces ground to provide preload when installed. To accomplish this, a gap is provided between the inner races or outer race of the pair.

There are three types duplex bearing arrangements commonly used and they are Back to Back, Face to Face and Tandem bearing arrangement.
It is a type of duplex bearing arrangement where the two half are placed so that contact angle lines of the bearing diverge inwardly. And these contact angle lines form ‘O’ shape, hence called as “O arrangement”.
In back to back arrangement a gap is provided between inner races and when installed, this gap is closed and producing a preload on the bearings. DB arrangement has higher moment stiffness.
They can carry heavy radial loads, combined radial and thrust loads, reversing thrust load, moment loads.
It is a type of duplex bearing arrangement where the two half are placed so that contact angle lines of the bearing converge inwardly. And these contact angle lines form ‘X’ shape, hence called as “X arrangement”.
In face to face arrangement a gap is provided between outer races and when installed, this gap is closed and producing a preload on the bearings. DF arrangement has lower moment stiffness, so they must be spaced sufficiently to provide resistance to moment load.
They can carry heavy radial loads, combined radial and thrust loads, reversing thrust load.
It is a type of duplex bearing arrangement where the two half are placed so that contact angle lines are parallel. This arrangement makes a very heavy duty thrust bearing arrangement, but it will not carry any radial load and accepts thrust loading in only one direction.