Small stitches, usually about 1 mm in length that “lock” the stitches in the fabric to prevent the stitch from unraveling when the thread is trimmed. Without Tie off stitches the thread can potentially unravel and destroy the embroidery. They are particularly necessary when making satin stitches wider than 1.2mm, as satin stitches have a tendency to unravel more easily than running or fill stitches.