|This illustrates the 45 degree angle of a T-shirt neck band as recommended by Marcy Tilton in this article in Threads Magazine.
Cutting the band this way eliminates the bulk of a straight seam that is folded in on itself at least once in a single layer band, or multiple times in a neck band that is folded lengthwise before it is attached to the garment neckline. It also helps to offset the seams, no matter which way you create a neck band, so that they don't end up on top of each other.
I've illustrated it in the photos as a folded band, but it can also be used as a single layer neckband.
I used it in this T-shirt for the first time and liked it very much. After sewing the band together and folding it lengthwise, I applied it to the neckline of the T-shirt following the double needle method I wrote about in this tip.
That whole Threads article by Marcy Tilton is a treasure trove for anyone who wants to sew refined T-shirts!