I grew up with metric measures, but it wasn’t until I started working with dimensional lumber that I came to appreciate the value of the imperial system of measures. Some people believe that just because something comes in multiples of 10, that it must be inherently better. I don’t believe so. Let’s take the case of the humble 2×4, the standard piece of building lumber in North America. In Australia a similar piece of lumber would be classified as 45mm ×90mm. The reality is that in metric measures, everything is typically expressed in millimetres. The problem is that mm are so small that it is hard to visualize, it is much easier to understand what an inch is. Go to a lumber store and order a 2×4×8, and everyone understands what that is.
The humble 2×4 is of course not actually 2 inches by 4 inches. This is the nominal dimension used when a tree is cut into dimensional lumber after it is felled. Dry the wood in a kiln, and it shrinks, less so in length, but considerably in the other two directions. Then the wood is dressed, by planing. The result is that a 2×4 becomes a 1½×3½. Easier to remember 2×4 than 1½×3½ though. Other dimensions of lumber dry differently. Typically a 1×6 is actually ¾” by 5½”.
In the early years of the lumber industry in North America, each region had its own specifications for lumber size, largely because lumber was sourced locally. As lumber began to be produced for different markets (e.g. domestic cities, export), a standard had to be considered. The Forest Service of the U.S. Dept of Agriculture produced an article in 1964 titled History of Yard Lumber Size Standards. It discusses the historical aspects of lumber dimensions, and is quite complicated. In the early 1900s, the common thickness for a dressed 1″ board was 13/16″. By 1929 this had changed to a standard 25/32″. Then on April 30, 1956, the American Lumber Standards Committee voted to establish ¾ inch as the standard dressed dry thickness, where it has remained to this day (there was an attempt in 1964 to modify the thickness again to 5/8″).