What makes a 2 x 4

Why is it that a 2 x 4 x 8 stud is actually eight feet long, but is not a 2 x 4-inch piece of wood?

In the old days, someone decided that two inches would make a good measure. The trouble was that each mill used its own gauges and there was no standardization, so rough-hewn lumber was approximately two inches thick, plus or minus half an inch. Then it dried, and with drying comes shrinkage. Today, every so-called 2 x 4 measures 1 1/2 inches by 3 1/2 inches. Woodworkers refer to nominal measure (what it’s called) versus actual measure (what it really measures).