Boards 1" or more thick are sold by the "board foot" which is equivalent to 12" square and 1" thick.

Boards less than 1" thick are sold by the square foot, face measure.

Dressed lumber comes in sizes 1/8" less than sawn lumber.

Regular sizes are: 5/8" dressed to 1/2" 3/4" dressed to 5/8" 1" dressed to 7/8" 1-1/4" dressed to 1-1/8" 1-1/2" dressed to 1-3/8" 2" dressed to 1-7/8" Any of these may be dressed down to thinner boards, or resawn on a special band-saw.
In ordering it is common to give the dimensions wanted, in the order of thickness, width, and length, because that is the order in which dimensions are gotten out.

E.g.: 6 pcs.quar.oak, 7/8" x 6" x 3'0" 2 pcs.quar.oak, 3/4" x 7-1/2" x 15" If a piece wanted is short the way the grain goes, the order would be the same, thus: 3/4" x 11" (wide) x 6" (long).