Thanks all-—I always thought a 2/4 was a 2/4.
What I don’t understand is how this decision can stand if the sizes are the same at all lumber dealers.
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You and me both. It shouldn't stand. Nominal sizes are used everywhere, not just Lowes. It's handy when describing wood stock because actual sizes can vary. For instance, plywood varies in actual thickness depending on grade and what grade each side has. You can have grade B on one side, and grade C on the flip side depending on how nice you want your finished project to look on the outside versus inside. The B grade is smoother, hence more milling than the C side. So 1/2 inch nominal can be 15/32 inch or 7/16 inch or whatever. Much easier just to label it with the nominal size and the grade.