When a sawmill cuts a log up into dimensional wood, they are cutting green, unseasoned wood. That means that the lumber has a higher moisture content.

During WWII, the war effort demanded large shipments of dimensional wood that was green and cut into specific sizes. That means that a 2×4 piece of lumber was cut at the sawmill from a green log, and measured 2” by 4” at the time of cutting.

However, as the wood dries out and cures, it shrinks. That means that the 2×4 cut green doesn’t meet the same dimensions after it’s cured due to shrinkage.

And that is still how we do things today.

Dimensional wood is cut green to the common dimensions we use today. When it is cured, it reaches the actual size. Lumber standards actually recognize both green and cured lumber, and have size.

we are now that much more mature for knowing this