Tongue and Groove Joinery

Next, change your stacked dado blade set on your table saw to a 1/2" width, and adjust the depth of cut to about 3/16". Clamp a clean, flat sacrificial board onto the fence (making sure that your clamps are at least an inch off of the table surface), and then slide your fence over so that the sacrificial board is just barely touching (or just a hair away from) the dado blade.

Locate the tongue side of each board from your layout and lay one side of the board flat on the table and against the sacrificial fence. Turn on your saw and push the board completely through the blade. This completes one half of the tongue. Then, flip the board over (so that the opposite face is against the table), hold it against the sacrificial fence and cut the opposite side of the tongue.

If all went according to plans, you should have a tongue that is a little too fat to fit into the corresponding groove. This is by design. Determine how much more you need to take off (conservatively), and raise your blade a little higher before running both sides of the tongue through the blade and testing the fit again.

Continue this procedure until your tongues fit snugly (but not too tightly) into the grooves. There should be no play, but the fit should not be so tight that you have to force the boards together after you apply glue (in an upcoming step).

TIP: To ensure that your board stays flat on the table and securely against your fence, you might consider clamping a featherboard against both your fence and your table to hold the board in place across the blade.