As nuffsaid, said...they are different thicknesses. If you have a highshank domestic machine, you may need 1/4" rulers which you would then use on your mid arm, but if you have a low shank, which is more common, then you would have to buy 1/4" height rulers for your midarm.
Just find a Westalee dealer and tell them what model of domestic machine you have and they will tell you exactly what foot and rulers height you need.
Watson