Question: why are 2 by 4s called that?
Answer: In the past, when a timber was called a 2x4 [or "two-by-four"], it actually measured 2 inches by 4 inches. ... Because of this extra milling, a 2x4 no longer measures a full 2 inches by four inches. Instead, a 2x4 is really only 1 1/2" by 3 1/2".