As I was reviewing my Trouser Draft CD for updates, I wanted to revise how I treat a bag under the seat. So, for those who have my CD, and those who don’t, here’s the revised way I treat this alteration.
The illustration shows the problem—ripples under the derrière. This occurs for a variety of reasons, but it can be easily corrected.
Alter muslin then transfer the changes
Pin the excess out of the muslin, under the seat, so the trouser leg hangs crisply. This will result in a double-ended dart shape. Transfer this shape from the muslin to the paper pattern.
Add lines to the pattern
After the alteration is on the paper, draw two parallel lines, 90 degrees to the grainliine. The distance between these lines, is the deepest part of the dart. Measure the distance between the two lines.
Next, fold out the excess by bringing the two parallel lines together and taping them closed. The shaded area represents the amount of excess folded out. Making this fold causes the inseam and outseam on the back leg pattern to be shorter.
Now comes the revision
To correct the back pant-leg length to again match the front side-seam and inseam lengths, add the length you removed back at the hem of the trouser. The trouser will now sew together correctly without bagging under the seat.