The single biggest factor in whether a necklace looks right isn't the necklace at all, it's the neckline underneath it. Matching the two correctly makes any piece look intentional, and getting it wrong is why a necklace you love can sometimes look off in the mirror.
Crew neck and high necklines
Crew necks and other high, closed necklines create a natural frame right at the collarbone, so pendant necklaces in princess length (17-19 inches) work best here, sitting just below the fabric line rather than disappearing under it. Longer matinee or opera lengths also work well over a crew neck, since they fall onto open fabric rather than getting lost.
V-neck
V-necks are the most forgiving neckline for jewelry, since the open V naturally draws the eye downward and most lengths look good against it. A pendant that echoes the angle of the V, sitting a little below where the fabric meets, is a classic, flattering pairing.
Scoop and boat necklines
Wide, shallow necklines like scoop and boat necks pair beautifully with shorter, closer pieces such as chokers, since there's more exposed skin at the collarbone for a shorter necklace to sit against without being cut off by fabric.
Off-the-shoulder and strapless
These necklines expose the most skin around the neck and shoulders, which is exactly why chokers were made for them. A choker draws attention to the collarbone and neckline itself, while a delicate princess-length pendant can add a second, softer focal point without competing with a bold neckline.
Turtleneck and high collars
Skip short necklaces here entirely, since they'll be swallowed by the fabric. Opera-length or rope necklaces worn over the collar are the classic pairing, adding a long vertical line that elongates the silhouette rather than getting hidden.
Button-down shirts and collared tops
A layered look with two lengths, one shorter piece sitting just above the collar and one longer piece hanging below it and inside the open buttons, tends to look the most deliberate here, since it works with the structure of the collar rather than against it.
The one rule that always applies
When in doubt, princess length (17-19 inches) is the safest, most versatile choice for almost any neckline, which is exactly why it's the length most classic pendant necklaces are made in.