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:
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Source: https://xkcd.com/3138/
Unfinished 2x4s are actually 2in x 4in, which originally you would trim on all sides to get a nicer surface getting down to the "modern" dimensions. Nowadays, you pretty much only see pre-trimmed lumber.