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Every woodworker knows not to rely on inside measurements made with a tape measure. Instead, I use two steel rules with dimensions that run to the edges. I butt one rule at each end, so the graduations overlap. To calculate the exact inside measurement, I read one ruler from the 10-in. mark on the other and add 10 inches. Using this method, the inside measurement above is exactly 15-1/2-in. –Serge Duclos
