A princess length necklace is 18" long and usually refers to a string of pearls.
                                                            
                                                     
                        
                                                
                        
                        
                            A necklace that is 18 inches in length.
                                                            
                                                     
                        
                                                
                        
                        
                            A princess length necklace (usually referring to a string of pearls) is 18" long.
                                                            
                                                     
                        
                                                
                        
                        
                            Used to describe a chain or single strand of pearls or beads that is about 18" long.