Used with beading thread (i.e. Silamide, Nymo, fine cord).  As the needle is joint at the top and base only, it has a very large, flexible opening where your thread feeds through, and they collapses to help hold your thread in place.
Especially good for working threads that are to thick to use with your ordinary beading needles with seed beads as small at size 12.
4 needles per pack