Quote Originally Posted by Peelee View Post
I fail to see the problem. I'm assuming that the holes are designed around the standard lumber size. Making the standard lumber size the same as what it's named for would make the standard hole size bigger by default. Its not like there are natural construction holes these naturally fit into. It's all made by design. So saying, "the system we made specifically for it would be different if we made it specifically for slightly larger than it" is correct, yes, but also kind of meaningless.
Concrete example: I have a 4 inch gap I need to build a block to fill. It's probably not exactly four inches. If my 2x4s are exactly 2x4, then if that gap is a hair under 4 inches, I have to rip one of those boards. That in turn requires a large tablesaw, is a dangerous and fiddly operation requiring additional measurements. If my 2x4s are under nominal, I need to stick a shim in the hole to fill the gap, which is much easier to do fine adjustments for. Now imagine doing layout, roughly 4 inch gaps will be a lot more common than roughly 4/12 inch gaps.

Sure, one could do layout so that your gaps would tend to end up at 4 1/2 inch thickness. This will tend to mean dealing with lots of stupid things like 11 3/4 inch lengths. In return I get what? The ability to have 2x4s that are actually supposedly 2x4. The only person this could possibly help is somebody who has never dealt with lumbar before. Except it only really helps them if they designed something assuming that you could just take a 2x4, nail it to another one and get something exactly 4x4 inches.

And that's a bad habit that will screw them over very quickly, regardless of the nominal thickness. If you're going to be successful working in wood in any capacity, you need to know not to count on across-grain thickness being stable, because it changes. Every time you cut the board longways, you will relieve internal stresses, causing the wood to warp and change shape. Every time the humidity changes, the wood will change size, mostly by getting thicker not longer. If you need exactly four inches thick, you need to spend a lot of work getting that, probably over the course of days as you allow the wood to relax after successive cuts. Even that may not be enough, I had a box lid pick up a substantial warp after being allowed to dry for more than a month. So whatever nominal thickness the mill cuts to is not the one you will get. I don't count on a 2x4 being close enough to 1.5 x 3.5 inches to be remotely reliable as a measurement, if I need a piece of wood of exactly that, or any other measurement, I'll get a bigger one and cut it to that size.