The idea of nominal and actual dimensions has been a consideration in the millwork industry for years.
Nominal Dimensions
The nominal dimension of a piece of lumber refers to the size of the board when it was rough cut.
Actual Dimensions
The actual dimension is the final size of the piece of lumber after it has been dried and planed.
Why Two Measurements?
When preparing a piece of wood, it is initially cut to a certain size, for example 2″ x 4″. This is the nominal measurement of the piece of wood. After cut, the wood is dried planed/smoothed, which results in the lumber shrinking slightly to 1-1/2″ x 3-1/2″. The final size of the piece of lumber is the actual measurement of the piece of wood.