This 6" ruler is a classic and essential sewing aid. It has so many uses.
The slider allows you to set a hem depth, and use it as you move along the garment with the iron.
You can use the notches on one side of the ruler to mark out even inches.
You can use it to determine a specific width to be marked on your sewing machine with tape.
and more...
It's an essential in any seamster's sewing area.