Can anyone explain to me what the four basic NHLA grades are for hardwood?
Take a look at this article, Understanding Hardwood Lumber Grading
The best grade is FAS and it must be 83% clear or clearer on the poorest side. Clearness is measured in large rectangular areas called cuttings.
The next grade is No. 1 Common, which must be 67% clear on the poor side, etc.
Next is No. 2 Common, which is 50% clear. Then No3A Common, which is 33% clear. Then No. 3B, which is 25% sound.
There is also a grade called Select (which includes another popular grade called FAS 1-Face). Selects are FAS on the good side and No. 1 Common on the poor side.
There are many other restrictions, including number of cuttings, size of cuttings, and size of lumber.
Gene Wengert, forum technical advisor