My Mom likes to tell a story about how my Dad was explaining a complicated theorem in topology to her. At the end of the explanation she replied “Oh yes. Those are called darts.” When she taught me to use a sewing machine last year, she tried to encourage me by saying “Sewing is just applied topology.” Of course, my mom got a PhD in math, so presumably she knows something about topology. Me, not so much. It might have come in handy this past week.
I had the idea that I would make two jackets this month. One with a hood and one a bit more fitted. I was going to make them from the same pattern, V8541, which I have made once before so I didn’t think it would take too long. Here’s what I had in mind for the more fitted version:

And here’s the pattern I started from:

(from the Vogue Patterns website) except with a curved back seam I put in the last time I made this jacket.
To convert the darts to princess seams, I sliced the pattern pieces through the darts, and rotated the side of the front pattern piece to close the dart. I also transferred the lump of my upper back (shaded area) to the back side piece, so I could cut the center back piece on the fold:

I made my first muslin and sewed it up. I thought it fit pretty well and was very pleased with myself, until I took a picture of the back:

What’s up with the seam on the right? And why are the sleeves pulling in from the sides?
The problem with the sleeve is that I tried to alter it with no idea of what I was doing. I read on Dawn’s blog that if I added fabric to the bottom of the armhole, it would give me more range of motion. That seemed like a good idea, so I went ahead and added about 1/2 inch. Then I figured I should add the same amount to the parts of the sleeve that matched. And then the sleeve cap was too big so I took a little off the top of it. After looking at my weirdo muslin though I did the sensible thing and googled “sleeve alteration.” Google directed me to this site, which explained that the proper thing to do is to add fabric to the sides of the sleeve caps, and make the sleeve narrower:

Why is that the right thing to do? Well, I made the armhole smaller so the circumference of the sleeve has to be narrower to match. Since I didn’t change the circumference of my sleeve I was trying to stuff a wider sleeve into a narrower hole. Hence the pulling.
Here’s my second muslin with the sleeve fixed on the left:

I was still perplexed about the back seam. Part of the problem is that I think my shoulders are asymmetric. I spent several days tweaking the jacket trying to made the sides look even. Then I cut the muslin apart and went to go trace the pieces. Since I was out of tracing paper I copied them onto my dress sloper.

When I lined the front jacket pieces (which fit well) up to the center front of the dress I saw that the bust points were at the same level, but that the jacket dart was little to the side of the dress dart and the jacket side seam was a bunch further to the side. That made sense because the jacket has to go around whatever I’m wearing underneath. Its circumference is wider than the dress’s so if I line the two up at the middle, all the points to the side should be further out on the jacket. On the back, though, the jacket dart fell on top of the dress dart. Eureka! I adjusted the back to match (roughly) the proportions on the front. The dress sloper also let me see what jacket length would be most flattering. After a few more tweeks, here’s my final muslin (still haven’t fixed the sleeve on the right):

The smile is because I feel like I just solved a particularly tricky problem set. Its only a half smile because I realize I’ll have to do this again for the hooded version.