In knitting, a yarn over creates a hole and increases the number of stitches, making the piece wider. In order to perform a yarn over, you must have knitted at least one complete row of stitches. With the finished stitches on the left needle, complete a knit or purl stitch as normal. Instead of progressing to the next stitch, wrap the yarn around the needle from back to front, and this is the yarn over. Complete the next stitch as you normally would. When you progress to the next row of stitches, treat the yarn over as if it were a complete stitch.
Yarn overs between two knit stitches or two purl stitches are performed this way. Performing a yarn over between a knit stitch and a purl stitch, however, is a slightly different process. Instead of completing stitches at the back of the needles, you make them in the front of the piece, the side facing you. To complete a yarn over when working with purls, wrap the yarn around the needle twice from back to front. This keeps the yarn on the right side of the needle.
Use caution when knitting or purling with a yarn over, because the looped yarn will be very loose and easy to drop. If you drop the yarn over and can see the yarn clearly, catch the loose thread between your fingers and replace it on the left needle. Otherwise, it might be necessary to undo the last two rows and start over.
A yarn over can be used to make a decorative pattern such as a row of eyelets for lace work. The hole created by a yarn over can also be used as a buttonhole. If working with fine material, such as fingering weight yarn, consider reinforcing the buttonholes with embroidery floss.