The Lumber Industry has an "interesting" way of measuring wood.
When you buy a 2X4 it is (most likely)
really 1.5 by 3.5. This is because lumber is named by its rough-sawn "nominal" size, then planed smooth and dried down to the smaller actual size.
This is a
good explanation of how to measure board feet. Another
list of the actual measurements of wood. Nominal is the size they call it in the store; Actual or "Dressed" is the real size of
the wood.
My experience with Treated Pine is as follows:
Dressed
Nominal
0.75" X 3.5" = "1 X 4 X (length)"
1" X 5.5" = "5/4 X 6 X (length) Treated Y/P Std/BTR
Decking"
1.5" X 3.5" = "2 X 4 X (length) No 2 Prime PT SYP .40"
1.5" X 7.25" = "2 X 8 X (length) No. 2 Prime PT SYP .40"