Piping foot #38 is often also termed the "uneven foot", owing to the different thicknesses of the two halves of its sole. Consequently, this foot is ideal for creating narrow-to-medium piping, as well as for sewing unevenly thick fabric layers.
The right side of Piping foot #38 is higher than the left side, allowing it to ride easily over the fabric layers as you sew. In addition to piping, this foot is also suitable for sewing on thick tape and ribbon, cords or hook-and-loop tape.
TIP: when sewing on tape, the tape should lie under the right side of the foot, flush with the edge of the thicker left side. This will allow the needle to catch the edge of the tape more easily.