What does ridges mean in knitting?
Each “ridge” is actually two rows of knitting. So, for example, in the Garter Stitch sample pictured, there are four ridges of yellow, which is eight rows of knitting. The stitches, then, are the count of either the top or bottom part of a single ridge.
What is the purl stitch?
: a knitting stitch usually made with the yarn at the front of the work by inserting the right needle into the front of a loop on the left needle from the right, catching the yarn with the right needle, and bringing it through to form a new loop — compare knit stitch.