xkcd #3138: Dimensional Lumber Tape Measure
Title text:
A person with two watches is never sure what time it is, especially if I got them one of the watches.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3138/
Huh, the explain link says the dimensional sizes originated from the wood being cut at the listed size while green, then shrinking as it dried. I was told that it was done for construction purposes, where the wood would likely be covered by plywood or drywall that would bring the dimension up to size. I never questioned it before; that always seemed plausible enough.
Modern lumber is planed, so some of that difference is because of losses from that. If you open up the walls of a house built 100+ years ago, you see these thick rough wall studs that never went through a planer. Even with shrinking, it's close to being actual 2" x 4".