Board lumber is commonly available with either straight or radius edges. Both types come in a number of standard sizes.
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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).
Cheryl Ess has worked in computer science for over 35 years. She has done technical writing for both government and industry, including work for the FBI and for well-known businesses such as Anheuser-Busch, General American and Monsanto. During her shorter career in engineering she wrote EPA operating permits. Ess holds Bachelor of Science degrees in computer science and civil engineering.