Length is the main variable, not the pendant
Two necklaces with the same charm or chain style can look completely different depending on where they fall, so length is usually the first thing to decide when buying -- everything else, including metal color and pendant shape, is secondary to getting the drop right for the neckline you are pairing it with.
Layering rules of thumb
When stacking multiple necklaces, a gap of at least 5 cm between lengths keeps the strands from tangling and lets each one read as a distinct layer -- for example, a choker with a pendant length, or a pendant length with an opera length, rather than two pieces of nearly identical drop.
Frequently Asked Questions
Which necklace length works with almost any outfit?
Pendant/princess length (43-48 cm) is the most universally flattering, since it sits in the middle of the chest without competing with most necklines, which is why it is the most common length for everyday, single-pendant necklaces.
Is matinee the same as bib?
They overlap but are not identical -- matinee technically refers to a length, while bib refers to a silhouette that spreads wide across the chest. Many bib necklaces happen to fall at matinee length, but a matinee-length necklace is not automatically a bib style.