No jewelry is meant to be worn by itself, unless you’re Rose from the movie Titanic. Jewelry is meant to be worn with other accessories and wardrobe pieces and therefore must go with all the rest in order to create a harmonious, cohesive look or. This is especially true for necklaces.
People can notice the clothes but often miss the ring or the earrings, but necklaces are always seen with what one is wearing. Necklaces are the jewelry that is most spatially proximate to clothing, as they fall close to, often overlapping the neckline, sometimes covering the entire front, and thus and can most contribute to the success of an outfit or its ruin.
There are different types of necklaces to go with different types of necklines. A general rule is that a necklace should also complement the outfit, not clash with it. Another is that it should not be directly on top of neckline, and so should sit at least either few inches above or below it. An exception to this would be the bib necklace, which is wide and elaborate, and covers a lot of surface area, usually obscuring the neckline itself, in which case the only requirement would be that it be worn on a plain and simple top or bodice.
A collar necklace closes around the neck a little above the shoulders, while a choker sits on the base of the neck where it meets the shoulder. These are best worn with U- or boat- necklines, strapless outfits and collared ones. A princess length necklace falls halfway between the base of the throat and the cleavage while a matinee necklace falls right in the middle of the chest. These are best worn with high round and sabrina necklines. An opera necklace falls below the chest and anything longer than that is called, well, a long necklace. These longer necklaces go with any type of neckline but the top or bodice generally must be simple or at least not too embellished.
Just like there are necklines to go with certain body types, certain types of necklaces complement certain body types. Small-breasted women can look more busty with bib necklaces or layers of different lengths of chunky beads. For women with large breasts, long pendants on substantial chains are flattering. Those long Y-shaped lavalier necklaces have a kind of breast reduction surgery effect that you would otherwise have to get from a place like Finesse Surgical, as they cut down along the front to break up that wide expanse of chest. Any type of long necklace would have the same effect, but must have a pendant to weigh it down, otherwise it’ll fall on the sides of the breasts and would them look bigger instead.