Sterling Silver Box Chain, 1mm win width, Lobster clasp.  available is various sizes...
16" Choker---Sit at the base of the neck. Fits almost everything.

18" Princess---On the collarbone. The most common length for average women. 

20"-24" Matinee---Between the collarbone and the bust. Great for both business and casual wear.

30" Opera---On the bust or an inch or two below. Worn best with high neckline and evening wear.