T-square Biscuit Joiner Jig
Building with a biscuit joiner is such a pleasure. To make it even more enjoyable, I fashioned a special T-square from scraps of 3⁄4 " medium-density fiberboard (MDF). I use the jig when building bookcases or shelved cabinets. I mark the desired locations of biscuits on the long leg of the square, and cut a saw kerf at each mark. Those kerfs serve as registration lines for the joiner.
It's quick work to cut slots into the side of the carcase, then cut matching slots into the ends of the shelves. You may want to make different T-squares for projects of different dimensions.
–Glenn Willis, Jr., Redondo Beach, Calif.