It simply refers to how many 1/4-inches thick the rough stock is. For example, 4/4 (read out loud as four-quarters) is 1-in. thick in the rough. 5/4 is 1-1/4-in. thick, 12/4 would be three inches thick.

What does 5 quarters mean?

: the parts of a slaughtered animal other than offal that supplement the four quarters (as the giblets in poultry and the head, tail, hide, horns, hoofs, fat, tallow, tongue, heart, and liver in cattle and sheep)

What is the thickness of 5/4 lumber?

5/4 (five-quarter) lumber is 1.25″ thick when rough sawn. When 5/4 lumber is planed, typically it is finished at 1.063″ thick.