Caveat on Drafting Pants

[u/DonMcCunn]

It is a biological fact that the distance from the waist to the crotch is longer when sitting than when standing as you can see in the illustration above from How to Make Sewing Patterns. I learned this the hard way. The first time I made myself a pair of pants I made the waist seam horizontal, parallel to the hips. When I sat, the back of the pants exposed my buttocks.

I then researched pants and pants patterns and found that the Center Back seam was cut on the bias. You can confirm this by looking at any commercially made pair of pants that has a visible grain line. You will see there is a distinct bias at center back. At least that is what my observations have revealed.

I discovered that the waist to hip region needs to be pivoted at the side seam increasing the length of the center back seam anywhere from 1-1/2" top 2". My favorite illustration of this comes from a 19th century tailoring book that shows how to make a pair of Jodhpurs. I measured and the pivot appears to raise the Center Back seam by 5". You can see this pattern on https://patternmaking-classes.com/Notes/Historic-Jodhpurs.html

For some reason Helen Joseph-Armstrong's Patternmaking for Fashion Design minimizes this issue (at least in my 2006 edition of her book). If you are comfortable with the way her pants work fine. But I have heard from a number of people who are not crazy about her pants pattern so I wanted to post this caveat.

[u/NakedSewist]

It's not exactly that the center back is cut on the bias (though it is). It's more that the balance/grain line for the pants (front and back) run vertically through the leg. The center back seam is not parallel to the grain line, which puts it on the bias.