Methods To Construct Children's Furniture - Lumber
Board lumber which is nominally 1 inch thick will measure from 3/4 to 7/8 inch (usually 13/16 inch). It's actual width will be 3/8 inch less than it's nominal width, in pieces between 2 and 6 inches wide, in wider pieces the difference goes to 1/2 inch.
In the same way, dimensional lumber of 2-inch nominal thickness actually measures 15/8 inches. So you have to be very specific about the size you want, and also check the extent that the dressed lumber varies from the rough-cut size.