The internal classification of eurypterids within the Eurypterida is based mainly on eleven established characters. These have been used throughout the history of eurypterid research to establish clades and genera. These characters include: the shape of the prosoma, the shape of the metastoma, the shape and position of the eyes, the types of prosomal appendages, the types of swimming leg paddles, the structure of the doublure (the fringe of the dorsal exoskeleton), the structure of the opithosoma, the structure of the genital appendages, the shape of the telson and the type of ornamentation present. It is worth noting that not all of these characters are of equal taxonomic importance. They are not applicable to all eurypterids either; stylonurine eurypterids lack swimming leg paddles entirely. Some characters, including the prosoma and metastoma shapes and the position and shapes of the eyes, are seen as important only for the distinction between different genera. Most superfamilies and families are defined based on the morphology of the appendages.