The fabric selvage edge is one of the most important elements when it comes to aligning and cutting sewing patterns. Not only does it denote the fabric grain, it also serves as a means for measuring a fabric cut at the time of purchasing. The fabric's length is measured lengthwise down the selvage edge while the standard fabric width is determined by measuring from selvage to selvage.
Keep in mind that all fabrics have two selvage edges positioned lengthwise on each side. The selvages also serve the important role of stabilizing fabric edges by preventing them from stretching, loosing their shape or unraveling,
Depending on the fabric and its weaving process, a fabric's selvage edge is denoted by any of the following marks:
Writing and/or a contrast color strip. This type of selvage is most commonly used with basic-weave cotton fabrics since they have great color retention and are easy to print on. In this instance, the selvage edge is very noticeable and can include fabric care instructions and information on the manufacturer.
A denser weave or a weave pattern different from the rest of the fabric. When this selvage blends into the color of the fabric, it is a little more difficult to spot but should be fairly distinct when compared to the weave density of the entire fabric cut. This type of selvage takes up very little space along the edges which conveniently maximizes the fabric area you can work with.
Perforations provide another common technique to denote the selvage edge. The perforations are usually accompanied by a weave that is denser or has a slightly different structure than the rest of the fabric.
A dense fringe is used most commonly on sweater knits, twill fabrics, and bulky/textured wools. The fringe is usually accompanied by a denser weaving which maintain's the edges' structure and stability while also preventing them from unraveling.
As noted above, the selvage edge denotes the fabric grain and allows for correct sewing pattern alignment prior to cutting. Regardless of what type of grainline your pattern indicates (lengthwise, crosswise, bias), it is always aligned parallel to the fabric selvage edge as shown.