New 'old stock', pre packed bias binding.  The edges are folded already to give you a 12mm wide tape, this is most often used by applying to the armholes or other faced areas.  To sew onto the arms holes, apply to Right sides together, unfolding one edge of the bias binding and use the crease as a line to follow (its about 6mm fold).  Pin the bias binding to your arm hole,  stretching and finger pressing it to follow the curve.Sew all the way around your armhole.  Now fold the bias binding to the inside, pin in place and sew down.