What’s the difference between sewing with the grain and sewing on the bias of the fabric? Fabric has a straight grain and a cross grain; sewing on the bias means cutting the fabric so the seam is at a 45 degree angle to both.
A gown or undergarment in a 1920’s bias cut style can be fun to wear, especially if the material is satiny and you have a perfect figure. It has a stretchiness and clings to curves. On the other hand, some more recent styles with chevrons or diagonal stripes can either have some of the stretchiness of a t-shirt or feel vaguely uncomfortable.
Sewing on the bias can present some difficulties because of the stretchiness of the material. Zippers may be difficult to put in and seams may pucker. The cross grain can also be a little bit stretchier than the straight grain, so it may not hang straight without an added center seam.
Here is a flared skirt that looks like it might be bias cut because of the way the material drapes, but if you look at it more closely, you’ll see the seams are actually either on the straight grain or the cross grain of the fabric. The last diagram shows how to take a pattern meant to be cut on the straight grain and cut it on the bias.