Its never been straightforward in my memory. In imperial, if you asked for a piece of 4 be 2 (4x2) then sawn would be 4 inches by 2 inches. If you said PAR (planed all round) you got 3 7/8 inches by 1 7/8 inches.
Today’s almost equivalent is 47x 100 sawn or 44x94 PSE Timber = Planed Square...