I genuinely hate that I'm going to say this, but...
A 2x4 is 1.5" X 3.5"
It's not _approximately_ 2x4 or _slightly_ smaller than 2x4.
Not that this adds critical information regarding the article, but if you set out to work with 2x4s and assume they are 2" by 4", you'll be sorry. And yes, I have made that mistake before, but it's a mistake you only make once.
This is not correct. The standard construction 2x4 you buy at any lumber yard/home improvement store is specifically cut to 1.5" x 3.5". Of course wood naturally expands/contracts depending on the environment so it will never be exactly those dimensions, but quite close (+/- 1/32" I'd wager).
Likewise with plywood, most (but not all) plywood sold in the US is actually slightly undersized. 3/4" is normally 23/32" thick, 1/2" is normally 15/32" thick, etc. This isn't because the wood shrunk (though again plywood will slightly expand/contract depending on the moisture of the environment), it's because the manufacturers meant it to be a little thinner.
There are still manufacturing tolerances for both types of products, but the lumber mills know what they are doing, they know what their customers require and provide tolerances that match those. Just imagine one very common usage: a deck. You think deck builders would be happy if the dimensional lumber wasn't a) very consistently the same thickness and b) very consistently the same width? Otherwise you'd have a very bumpy deck that looked pretty irregular.
Wikipedia's page on lumber[1] is a bit more rigid:
Today, a "2×4" board starts out as something smaller than 2 inches by 4 inches and not specified by standards, and after drying and planing is reliably 1 1⁄2 by 3 1⁄2 inches (38 mm × 89 mm).
