In detached buttonhole filling, you don't pass through the fabric as you work the buttonhole filling. In attached, you do. Attached buttonhole filling gives you more scope for varying the placement of stitches, so that you can create effective filling patterns. The detached buttonhole, though, gives you a bit more room for dimensional effects, since you can take out part of your foundation fillings and fold the buttonhole area back on itself - or make petals or leaves that stand away from the fabric. See how it's done in this video from Needle 'n Thread.
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