In lumber terminology, a 1″ piece of wood is known as 4 quarter (4/4). This is because lumber is typically measured in quarters of an inch. So, a 4/4 board is 1″ thick, a 6/4 board is 1-1/2″ thick, and a 8/4 board is 2″ thick.
The reason for this system is that lumber is cut from logs. Logs are not always perfectly round, and they can vary in diameter. So, when a log is cut into lumber, the boards produced will also vary in thickness. By measuring lumber in quarters of an inch, the industry can ensure that boards of the same thickness are always available.
Here is a table of the most common lumber thicknesses:
| Thickness | Quarters | Nickname |
|---|---|---|
| 1″ | 4/4 | Four quarter |
| 1-1/2″ | 6/4 | Six quarter |
| 2″ | 8/4 | Eight quarter |
| 3″ | 12/4 | Twelve quarter |
| 4″ | 16/4 | Sixteen quarter |
Above is an example picture – from Woodworkers Resources.