What stitch is this?

Backstitch

Backstitch is a short reversal in the direction of stitching used at the beginning and end of each line of stitching to keep the threads from unravelling.

Bartack

A bartack is a line of tightly stitched, tiny zigzag stitches. It’s used to reinforce seams in strategic places where there is going to be extra stress. They are used on jean flys, patch pockets, webbing and handles, belt loops, etc.

Baste stitch

Baste stitch is very wide, loose stitch used to keep fabric in place until the permanent stitching is sewn. Baste stitching is often employed for trying on clothes for size adjustments and for keeping things like pockets in place while sewing more permanent stitches. Often the baste stitches are removed once the permanent stitches are in place.

Edgestitch 

Edgestitch is a line of stitches close to the edge of a seam or fold. Edgestitching is often around 1/8” (~3 mm) or less from the edge. The stitch length of edgestitching is often increased slightly from normal stitches for aesthetics. Edgestitching can be used both as a style element and as seam reinforcement.

Serge

Serging is a seam finishing technique done by a machine called a serger. A serger uses four spools of thread instead of a regular sewing machine’s two sources of thread. Serging is the most common way to finish seams. A common alternative to serging is using a zigzag stitch on a regular machine.

Staystitch

Staystitch is a straight line of stitching within the seam allowance that helps keep vulnerable parts of fabric from fraying or stretching while the pattern is being sewn together. Staystitching is often sewn by the armhole in shirts or along the top edge of pants.

Stitch in the ditch

Stitch in the ditch is a line of stitching directly over an existing seam on the right side of the fabric. This technique is used to hide stitches and is useful on collars, waistbands, and fastening down facings.

Topstitch 

Topstitch is a line of stitches that is visible on the right side of the garment. Generally it is preferred to sew topstitching with the right side of the fabric facing up so the needle impressions are expressed toward the wrong side. Stitch length of topstitching is often slightly increased for aesthetics. Top stitching can be used both as a style element (eg. the decorative stitching on back jean pockets) and as seam reinforcement.

Understitch

Understitch is a line of stitching near a seam that fastens the seam allowance to the lining or facing. It is used to help prevent linings and facings from showing on the right side of the garment and to make seams look crisp. Often used in curved areas like necklines and armholes.

Zigzag stitch

Zigzag stitch is a common way of sewing with knit fabrics and finishing seam edges. The sewing machine needle moves to the right or to the left in between each stitch creating a zigzag pattern with the thread. This stitch works well for knit fabrics because it is able to stretch when the fabric is stretched.