First theory- The 'Swing Coat' method
This theory is often found in modern commercial costume patterns, and can closely resemble a modern ladies swing coat. This is the easiest model to pattern yourself, providing you have a base pattern that fits your shoulders and neckline reasonably well (a sloper of course, is perfect). This pattern could be from almost any other garment since the pattern is only used above the armhole.
The pattern is laid out on to the pattern paper (or cloth directly) using this method: Align the center front of the base pattern to the edge of the paper (selvedge of the cloth), leaving room for a seam allowance along CF. Trace the neck, shoulder and armhole of the basic pattern. From the bottom of the armhole, draw a line equal in length to the distance from your armpit to the floor, plus a few inches, angling out to meet the other selvedge. Mark the length on center front (the distance from your neck to the floor) and connect this point with a gently curving line to the point attached to the armpit.
For more information on this pattern, see this web site:
Comments on this method:
Requires wide cloths that are not directionally patterned (including napped fabrics and stripes)