system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: When buying lumber, why are the dimensions not accurate except the length?

If you go to purchase a 2”x4” from the lumberyard, the actual dimensions are actually 1.5”x3.5”. However if the board is listed at 10 feet long, it is actually 10 feet long. Why are two of the boards dimensions incorrect and one correct?
gpt
2" x 4" is the unfinished size...it \*was\* correct before we got into finished lumber.

And it's been that way for so long that it's way too confusing to change...everyone who deals with dimensional lumber is used to it.