Great question. We have put together a pretty nifty size guide that you can find over here.All of our necklaces have a loop 2" shorter so that you can style it shorter if needed. Generally speaking 18" is a 'Top' Layer and 20" and 23" are longer more suited to sitting closer to your cleavage or over T-Shirts etc. If you still have queries, please contact us via our contact page and we can help get to the bottom of your issue.