Board lumber is commonly available with either straight or radius edges. Both types come in a number of standard sizes.
Straight Edge Thickness
The actual size of a board is smaller than its listed size. According to Voluntary Product Standard, published by the National Institute for Standards and Technology (NIST), the actual thickness of a finished straight edge 2X board is 1.5 inches (38 millimeters), rather than 2 inches.
The cross section of a straight edge board has 90-degree angles at the corners. A radius edge board is made by rounding these sharp corners. After rounding, each edge has a straight center portion with a smooth quarter-circle at each end.
Radius Edge Thickness
Lumberyards specify a radius edge board, which was made from a 2X straight edge board, as 5/4X because the straight center portion of the edge measures 1.25 (five-fourths) inches.
Radius of Edge Quarter-Circle
The difference between the 1.5-inch straight edge thickness and the 1.25-inch radius edge center thickness is due to the two corner quarter-circles. Each has a radius of 0.125 inch (one-eighth inch or 3.2 millimeters).