What is a 'seam gauge'?

A seam gauge is a small ruler that has an adjustable flange (or marker) so someone can keep a hem or seam even while pinning and/or sewing. It is used by sliding the marker (flange) to a spot marked with the seam number you want to sew and then sliding it along your fabric to make creases (iron), or hems without the iron (while pinning or sewing.) Most people prefer to use it with an iron, as this seems to be the easiest.