Stitch rate (or stitches per inch) defines the number of times per inch a stitch occurs, just as gauge expresses the frequency of tufts across the width. Stitch rate is the number of times an individual needle inserts a tuft into the primary backing as the primary backing moves one inch through the tufting machine. This is sometimes abbreviated SPI. Therefore 10 stitches per inch means that as the primary backing moved through the tufting machine, a single needle form 10 tufts or stitches.