When you make patchwork for a jacket, the finished piece will be more stable if pieced to or backed with a foundation fabric.  When I make such a garment, I usually use a light-weight muslin for this purpose.  Before I use it, I wash the muslin to preshrink it.  If I plan to piece to the foundation, I cut the foundations a bit larger than the pattern pieces to allow for any shrinkage from stitching.