What is the actual size of a 4x4?

A 4x4, typically referring to lumber, measures 3.5 inches by 3.5 inches in cross-section rather than the nominal size of 4 inches by 4 inches. This discrepancy is due to the milling process that smooths and shapes the wood after it is cut. In construction and carpentry, the term "4x4" is commonly used to describe this standard size of lumber.