Can I get a Quarter?
I get this question a lot from woodworkers new to hardwoods. "What does four-slash-four mean?"
Four "slash" four, written 4/4, is actually pronounced "four-quarter." It's a term held over from days of old. A quarter equals 1/4 inch or one "click" of the sawyer's blade. So, a four-quarter board (4/4) would be four clicks and produce a rough sawn board that is about 1" thick. (4 quarters = 1 inch) By that standard 5/4 = 1-1/4", 6/4 = 1-1/2", and 8/4 = 2" thick.
The Hardwood Grading Bureau (yes, that's a thing) allows 1/4" to be removed from the thickness of a board to allow it to be clean on both faces. So 4/4 will finish at 3/4". 8/4 will finish at 1-3/4".
Side note: The thinnest standard material available in hardwoods in 4/4. Anything thinner is surfaced down at some point after it leaves the mill.