No they aren’t- they are allowed by their respective grading agencies to be scant of truly 2” x 4” because they are most often dried after sawing, which causes them to shrink.
What do mean no one knows why? It’s to use less lumber. Back when there was unlimited old growth timber they used true dimensions at the mill, even extra dimensional, I’ve seen 2x4’s that are closer to 3x5.
Modern 2x4s are 1½x3½ and rounded edges and nobody knows why.
Finished 2x4s are smaller, but unfinished are true
No they aren’t- they are allowed by their respective grading agencies to be scant of truly 2” x 4” because they are most often dried after sawing, which causes them to shrink.
What do mean no one knows why? It’s to use less lumber. Back when there was unlimited old growth timber they used true dimensions at the mill, even extra dimensional, I’ve seen 2x4’s that are closer to 3x5.