I cut the tongues for tongue-and-groove joints with a router. There are faster methods, but the router’s precision depth adjustment produces a fit that’s unbeatable. First set the router depth by trimming the edge of a scrap board. Flip the board over, trim the other side and test the resulting tongue in the groove (which has been previously cut). Make fine depth adjustments and continue to rout test tongues until the fit is perfect. To cut the tongue, first measure the distance from the router base to the bit. Then clamp a fence to the work this distance from the tongue. Gently tap the fence into perfect position with a mallet, checking the measurement with a steel ruler.
Jeffrey Cooper, Portsmouth, N.H.
Fine Woodworking Magazine, February 1981 No. 26