Also called Mangalsutra, Thaali, Thirumangalyam
The necklaces — usually several layered together, of which one carries ritual weight.
The mangalsutra is tied by the groom during the ceremony and, like sindoor, is a rite that helps constitute the marriage rather than an ornament worn to it. The black beads are traditionally understood as protective.
In Tamil tradition the thaali or thirumangalyam is tied with three knots; Telugu weddings use two mangalsutras, one from each family; Maharashtrian mangalsutras use two cupped gold pendants called vati. The forms differ so much that they are best understood as the same rite expressed differently.
Solah Shringar is a cultural tradition rather than a requirement — lists differ by region, and including or omitting any item is an ordinary family choice.