The back without folds is the most common cuts for shirts.
There are two types of shirt with back without folds:
The simple back is slightly curved on the side seam, fitted but not too much, it is ideal for wearing the shirt with a jacket.
Back with two darts hanger in kidney levels and follows the curve of the back. It marks the shoulders width for men and waist for women. It is just as elegant as the shirt with the simple back. This is the most current cut nowadays.