Single Width Single Inlet fans are provided with duct connections on both inlet and discharge. They are used on both ducted supply and exhaust systems.

Double Width Double Inlet fans are provided with duct connection on discharge only. They are used on both supply and exhaust systems and are usually installed in a plenum chamber or apparatus housing (Air Handling Unit).

The designated motor positions never change, regardless of the direction of rotation

PICTURED HERE ARE SOME TYPICAL FAN ARRANGEMENTS.
1. Arrangement 3, Double Width
2. Arrangement 7, Double Width
3. Arrangement 9, Single Width
4. Arrangement 4, Single Width 1