herrwood,
When you are talking about 1" thick boards being 3/4", you are thinking of 4s boards. That is to say that they are already finished (jointed and planed to a nice smooth surface) on all four sides like you buy at Lowe's or Home Depot or a lumber yard.
When you talk about 4/4 boards (USUALLY) you are talking about rough cut wood which will vary from 1-1/16" to 15/16" thick so that a person can finish them himself with a planer and jointer. A 5/4 board would be around 1-1/4" thick and be rough cut and need to be finished.
As far as buying a 4s board of 1" thick (which would be a 5/4 when rough cut) I don't think you can find one any where unless you pay a mill operator to finish one out for you.
I know this seems confusing and unfair (If I pay for a 1 x 6 board, darn it I want it to measure 1" thick and 6" wide!) But, the truth of the matter is that your 1x6 will actually measure 3/4" x 5-3/4" (if not 5-1/2") That is just way it is!?
I live in a fairly old house (built in 1953) The studs in the walls measure 2" x 4". I added a bedroom to the house in 1977, the studs in those walls 1-3/4" x 3-3/4" and today if you buy wall studs, they will measure 1-1/2" x 3-1/2"! It just don't seem right does it?
Rog