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I cut along the grainline (you can still see my penciled arrow) from the hem up to the edge of the seam allowance at the shoulder seam. I separate the pattern the desired amount, 1/2" in this case, and then tape in some scraps of my Swedish tracing paper. In order to make the pattern lay flat I make horizontal slashes, again only going to the edge of the seam allowances, near the top of the pattern piece.
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An overlap will occur along the horizontal slashes when the pattern is flattened out, and I tape them down. This process, however, distorts the shoulder line. A strip of tracing paper is taped to the top of the pattern piece and the shoulder seam is "trued up".
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Here you see the small amount of tissue that has to be added.
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Because my velvet, even from the Britex "bargain bin", is quite expensive, I really felt that I needed to remake my muslin. So I ripped it apart, cut new side fronts using the adjusted pattern and sewed it back up again. This is a truly miserable task, the less said the better. I just kept telling myself it would be worth the aggravation in the end.
So here's the revised muslin. Not really much of a change visually. It's still a very fitted garment, just much easier to "button". The change is more in how the jacket feels when worn.
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Not quite as much pull from the armpit across the chest, which was the goal.
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Unfortunately, I'm not quite sure what to make of the back. My hope is that the tablecloth fabric is clinging both to the felt shoulderpads and my cotton undershirt, thereby making these creases at the shoulder. Maybe this will all smooth out with my velvet and a silky lining? Of course what I'm dancing around here is that I may need a sloped shoulder adjustment. I've done it before. The big question is ... Do I have the drive to go there? It requires changing the shoulder line, dropping the armscye and adjusting the sides. A fair amount of redrafting. Muslin #3? I think I need to ponder this for awhile.