Originally posted by tim60 Because that is what it is, even when measured in other units.
You're right, of course. Except that a purchased 4x2 measures 90x45mm, where a true 4x2 would be 102x51mm. But that's rough sawn, of course, where the dressed size is 90x45mm. (It used to be 94x47mm, but has since be standardised down to 90x45mm) And we'll talk of an eight by two which is actually 190x45mm, not 203x51mm.
Plywood is interesting. Typically, a sheet of plywood is 2440x1220mm. Which is of course 96x48 inches, or 8x4 feet.
Doors are 1980 x 910/860/810/760/710/660/610mm which is odd. Everywhere else in the world as far as I can determine, doors are 2032mm tall, which is 80 inches. And, 813mm is 32 inches, 762 is 30 inches, and so forth. So, our door widths derive from inch measurements. But the height, in New Zealand is odd. 1980mm is almost 78 inches. So why we have 2 inch shorter doors than the rest of the world I don't know and I have been unable to establish. Old houses (from 1900-1920's had 80 inch tall doors. Yet, no-one can tell me when the powers that be decided to shorten doors with 2 inches.