There are basically three (3) types of mortisers. They are chain, chisel and reciprocating router. Chain mortisers are very fast however tend to leave breakout around the shoulders of the mortise, which may not be a problem for some applications where the tenon/mortise is not visible. Chisel mortisers are generally much less productive, however do produce perfectly shaped mortises with square shoulders and corners. These are popular in applications where there is no oversize around the tenon that will marry with the mortise and very clean shoulders and square corners are important as the joint is visible. The most popular style in use today are reciprocating router mortisers. These machines produce elongated mortises by reciprocating the rotating router bit back and forth as it penetrates the surface of the material. These mortises have clean shoulders, however due to the round router bit they do have rounded ends. These are generally matched to tenons with the same rounded ends