One thing that confused me when I just started sewing was figuring out what the selvage and the bias edges were. Sewing patterns would say “cut on the bias” and I had no Idea what that meant or why it mattered. I soon figured out (the hard way) how important it is to cut fabric as the pattern says- cutting pattern pieces on different angles can dramatically change the resulting structure of a garment. Have you ever had a shirt that kept twisting sideways while you wore it? This was most likely the result of a cutting error.
The selvage refers to the edge of the fabric that is tightly woven to prevent fraying- the sides of the fabric that do not get cut at the fabric store. If you look at the picture of the fabric below, you can see that one end is a lot more tightly woven and it even has a stripe along its edge that is lighter in color than the rest of the fabric- this is the selvage edge. You can also see that the other side of the fabric has frayed a lot more. Most pattern pieces are laid out parallel to the selvage- this ensures that the garment will hang off the body properly when worn.
Keep in mind that it’s a lot easier to find the selvage edge on woven fabrics.
Selvage Edge for Knits
For knits, you won’t be able to use fraying as an indicator (since it doesn’t fray). But not to worry- in most cases, the manufacturers add a marking or some contrasting thread to the selvage edge.
The bias refers to the 45 degree angle taken from the warp and weft of the fabric (see image below). You likely have seen bias tape in sewing and craft stores. This is made using fabric that was cut along the bias (hence, its name).
Cutting pattern pieces on the bias allows more draping creating a body-skimming effect in the finished piece. Also, the reason it is used for bias tape is because being cut on the bias allows the strips to follow curves more smoothly.
The Mood Guide to Fabric and Fashion (2015).