i think all our machines come with it, we just do not use it lol. i have one with my Pfaff and my old Janome, and they just call it a sewing guide for those long straight lines, instead of marking them with a ruler. i think it would be good for doing circles too once you have one drawn, then you just follow it with the guide and go on and on etc
oh i know what it is called now, cross hatching, at least in the quilting world that is what i call it
carman