It drills square holes and with a cross slide vise it cuts perfect mortises. I used it to build my kitchen cabinets out of pecan wood that had air cured for 17 years.
That was the hardest wood I've ever seen and the bitts cut it like it was balsa. This is the second set I have owned. The first set is in my shed somewhere, and I couldn't find it. I couldn't stand trying to make mortice and tenon joints with out one so now I have two.