19x26.5x2.5mm gold plated brass necklace shorteners (0.7x1x0.1"). Double a long necklace (32" or longer) and use this clasp to hook the doubled necklace in the back. Makes a two-strand necklace half as long as the original. Normally used for "endless" necklaces, but may also be used on some types of necklaces that have a clasp. Necklace can be twisted before hooking.
I have used these shorteners for over 20 years. They are the only shortners I use because the ones with safety clasps break. The silver colored one works better because the plating on the gold colored one tends to stick.