Many times dollmakers want to create a cloth doll or animal that will sit without assistance, or as a shelf-sitter. Usually, the best way to do this is to use mitered corners to form the derriere. I use this technique quite frequently in my patterns.
Mitered corners are also used to create pillows or cushions.
If you are not familiar with how to design or sew a mitered corner, Beth Gavin has created a Craftsy blog post that explains how mitered corners can be used to give your outdoor cushions a new look.
However, Beth's information is just as useful when creating your dolls. Once you learn the basics, you will find it a useful technique for many of your home decor and accessory sewing projects.
Here is Beth's blog, Box It Up: 3 Steps to Making a Square Pillow Cover.