|10/22/05 I've been informed that the link to my sketch doesn't workanymore. I'll try to find it on my home computer when I get home in a couple of days, and or re-do it. Sorry about that...
For those of us who consistently need to add length in the center back seam of pants and at the same time shorten the inseam (the ever-popular clown-butt-adjustment), here's a technique that accomplishes both alterations at the same time:
Slash the pattern horizontally at about the middle of the CB seam and 3 to 4 inches below the crotch level. The slashes need extend only a few inches into the pattern. Now connect these slashes by cutting parallel to the grain line. You are removing - isolating- only the lower crotch/upper thigh portion of the pattern. Now slide the isolated area down the back leg. I overlap the lower edge about 3/4". You will leave a gap in the pattern above the crotch level. That gap is effectively lengthening the CB seam; fill in the gap with scrap tissue paper. The overlap below crotch level works like the "clown-butt-adjustment", shortening the back inseam. Making this change leaves the sideseam contour and the grainline unaffected. The photo URL, with any luck, will give you a feeble sketch of the alteration. If nothing else has worked for you, give this one a try; it has really helped me make better-fitting pants for myself and my DD.