B is for backstitching
Backstitching is used to outline or make definition when you embroider or as I do cross stitch. It is usually done in black, but not always, sometimes the backstitching does not follow the holes in the fabric you use so you have to pierce the fabric and make a small hole with your needle.
Backstitching can be dreadfully boring, just as sewing in yarn ends in crochet or knitting I haven't met a single person who actually likes backstitching, but it does make a cross stitch piece come to life so it is well worth it I think.
Here is Somebunny to love- Eskimo kiss without backstitch
And here it is with backstitch.