Where the ornament hangs on your chest (or the nadir of the chain, if you have no pendant) affects both the overall style of a necklace and the types of shirts it goes well with.
Short, choker-style necklaces can be worn with just about anything, short of a suit and tie. They look unusual underneath a dress shirt or other turndown collar, but it’s not necessarily a bad thing. On their own above a low collar, of course, they stand out on their own, and viewers can see the whole necklace.
Mid-length chains that end right around the base of the neck or the top of the breastbone are typical for unornamented metal chains. This leaves most of the links visible with an open shirt collar, or with a non-turndown collar like a T-shirt’s. On the other hand, it’s the most awkward length for a pendant, since the ornament will fall under the collar on some shirts, above it on others, and awkwardly halfway-covered on a few.
Long chains or cords are good for pendants, but often hide them underneath your shirt. The ornaments attached to them will generally only be worn outside very casual outfits, meaning you’ll need a V-neck or a few open buttons if you want the pendant to be visible.
As with the materials, remember that you can always swap chains and cords in and out — and, in the case of metal chains, you can even have a jeweler shorten them, though if you can avoid the expense by buying the right length in the first place it’s obviously preferable.
