Originally Posted by Macybaby

Possibly more than you realize LOL!! Start by taking measurements, you want three measurements plus one "eyeball",

width of top gap (basically 1/4" or 3/16") then the height, and then the distance from the front gap to the needle hole (this is most often 1/2" or 5/8") and then look closely to see if the needle hole is centered in front of the gap, or offset to the left or right, and if it's a large offset or a slight offset.

BTW - there are sets that don't have a closed needle hole in the hemmers/binder. I refer to them as "open toed" feet, have no idea if they have an official name.