I purchased a Greist 1Z made for a zig-zag machine on Ebay. It can make thirteen different size buttonholes with the cams included. Some buttonholers only come with four cams. But you can always buy more.
The buttonholer as seen from the front. The presser foot is taken off, and the buttonholer is attached with a screw in its place. Also notice the white feed-dog cover plate. The buttonholer moves the fabric back and forth, so the sewing machine's feed-dogs are covered.
The buttonholer does a great job. Here are several different styles done with it. The three thin looking ones to the right are the Brother's built-in ones. The Greist makes heavy-duty looking buttonholes.
No one seemed to have an easy explanation of what model to buy to fit my machine. The back of the Greist box explains it very simply.
Thanks for stopping.