PAR means "Planed All Round" ie the wood is planed rather than rough sawn. So it's ready for use. However the planing removes some timber. So, in more detail, if the nominal size is 2" x 1" (say), which in millimetres would be 50mm x 25mm, then that means the thickness BEFORE planing. The planer takes about 3mm or 1/8" off each side. Thus a "planed all round" (PAR) 2" x 1" is 6mm or 1/4" less in total. Thus it is 1 3/4" x 3/4" or 45mm x 20mm (approx, because planers vary a little) So to sum up, 2" x 1" is approx 45mm x 20mm, 3" x 1" is approx 70mm x 20mm, 4" x 1" approx 95mm x 20mm etc

And so it is the PAR size that you use, not the nominal sizes. The plans make allowance for the planing loss, so notches are 20 x 20mm for example