The S series irons create single lines. Instead of two edges like the F irons, the S irons have just one edge. You can create additional decorative lines on top of the leather with an S iron. You can also heat treat just the edge of your leather pieces and without the top line that you would get with the F irons. S irons are numbered according to the width of the single edge; the lower the number the thinner the edge width.
S Iron Sizes