To join an inward corner seam to an outward corner, you have to reinforce the inward angle first for about 2cm on each side.
Insert a pin diagonally across the point where the stitching form the angle.
Clip up to the point being carefull not to cut past the stitches.
Spread the clipped section to fit the other edge. With the clipped side up, stitch on the seamline and pivot (reverse stitch) at the corner.
Sewing the square neckline:
Reinforce the corners first.
With right sides together, pin the garment neckline to the facing and machine stitch together. Clip the corners as described above.
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