This is a fraction. It identifies the thickness of wood.
- 4/4 means approximately 1" thick
- 5/4 means approximately 1-1/4" thick
- 6/4 means approximately 1-1/2" thick
- 8/4 means approximately 2" thick
- 12/4 means approximately 3" thick
There are some nuances to the precision of this fractional name. So, read a more in-depth discussion in our blog if you're curious. .

These are fractions that identify the rough milled thickness of lumber. The actual size depends on if it's still rough sawn or if it's been surfaced.
You might find this article in our blog pretty useful if you want to know more: