Did you know 2 x 4's are not actually 2 inches by 4 inches? When the board is first rough cut at the lumber mill, it is a true 2" x 4", but the drying process and planing of the lumber reduce it to the finished 1.5" x 3.5" size. Below are the common sizes of lumber, and their actual sizes.
Nominal 
Actual 
Actual (Metric) 
1" x 2" 
3/4" x 11/2" 
19 x 38 mm 
1" x 3" 
3/4" x 21/2" 
19 x 64 mm 
1" x 4" 
3/4" x 31/2" 
19 x 89 mm 
1" x 5" 
3/4" x 41/2" 
19 x 114 mm 
1" x 6" 
3/4" x 51/2" 
19 x 140 mm 
1" x 7" 
3/4" x 61/4" 
19 x 159 mm 
1" x 8" 
3/4" x 71/4" 
19 x 184 mm 
1" x 10" 
3/4" x 91/4" 
19 x 235 mm 
1" x 12" 
3/4" x 111/4" 
19 x 286 mm 
11/4" x 4" 
1" x 31/2" 
25 x 89 mm 
11/4" x 6" 
1" x 51/2" 
25 x 140 mm 
11/4" x 8" 
1" x 71/4" 
25 x 184 mm 
11/4" x 10" 
1" x 91/4" 
25 x 235 mm 
11/4" x 12" 
1" x 111/4" 
25 x 286 mm 
11/2" x 4" 
11/4" x 31/2" 
32 x 89 mm 
11/2" x 6" 
11/4" x 51/2" 
32 x 140 mm 
11/2" x 8" 
11/4" x 71/4" 
32 x 184 mm 
11/2" x 10" 
11/4" x 91/4" 
32 x 235 mm 
11/2" x 12" 
11/4" x 111/4" 
32 x 286 mm 
2" x 4" 
11/2" x 31/2" 
38 x 89 mm 
2" x 6" 
11/2" x 51/2" 
38 x 140 mm 
2" x 8" 
11/2" x 71/4" 
38 x 184 mm 
2" x 10" 
11/2" x 91/4" 
38 x 235 mm 
2" x 12" 
11/2" x 111/4" 
38 x 286 mm 
3" x 6" 
21/2" x 51/2" 
64 x 140 mm 
4" x 4" 
31/2" x 31/2" 
89 x 89 mm 
4" x 6" 
31/2" x 51/2" 
89 x 140 mm 
