Diagonal creases from the fork of the trousers are caused by the top of the inside leg seam being too small.
Let out the top of the seam approximately ½” (13mm) (point A) and curve into the original line. Alter front and back patterns in the same way. it is usually also necessary to hollow the centre front and centre back seams slightly (Line A-B).
If the trousers now appear too wide around the thigh, they may be narrowed by subtracting the same amount from the outside seam at point C as was added at point A. curve the outside leg seam smoothly. The broken line indicates the original line.
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