An extender necklace is a type of jewelry accessory that is designed to lengthen the chain or cord of a necklace. It typically consists of a small piece of chain that is attached to the existing necklace with a clasp, allowing the wearer to adjust the length of the necklace to their desired size. This can be useful for adapting a necklace to different necklines, layering multiple necklaces, or simply ensuring a comfortable fit.