The Little Foot was designed as an aid for quilters to obtain consistent 1/4" seam allowance.
The right side of the foot is exactly 1/4" to the right of needle center. By aligning your fabric with this edge for every seam, you will achieve a consistent seam allowance.
The left side is wider to cover more feed dogs for better fabric control.
This low shank model will fit most models including a featherweight. Bernina machines require an adaptor and Pfaff Little Foot Low Shankmachines must disengage the duel feed device, remove the snap on shank and screw on the Little Foot.