A back stay serves the purpose of lending stability to the back of a coat, right at the upper back. It’ll also help keep the shape of your coat while you wear it and while it’s hanging on a coat hanger. You will want to add a back stay even if you’re underlining your coat.
Coat pattern pieces
Firmly woven sew-in interfacing, though you can also use a quilting cotton or muslin
Tracing paper or something equally suitable
Draft the back stay
Since the Spearmint coat has a side back panel, you will want to overlap your pieces before you start creating your back stay pattern piece.
Make a mark 7-8″ down from the neck along the back seam.
Make another mark 3″ down from the armhole.
Connect your two mark with a gently sloping curve.
Trace your back stay pattern.
Since the back is seamed, you want to cut off the seam allowance and mark your pattern to be cut on the fold.
Cut out your back stay in a suitable fabric/interfacing. Use pinking shears, pink the bottom curved edge so you don’t have a hard edge along the back.
Attach the back stay
Stitch up your back fashion pieces, right sides together. Press seams open.
Place your back stay on the wrong side of the back seamed pieces. Smooth out your back stay and machine baste around the side, armhole, shoulders and neck, leaving bottom curve free. You can also use glue and glue baste your back stay in just don’t glue the bottom edge. Let it dry and continue with construction.