| 1a. | Photophores present on sides and belly | Family Batrachoididae Toadfishes |
| 1b. | No photophores on body | 2 |
| 2a. | Body encased in bony plates | Family Agonidae Poachers |
| 2b. | Body not encased in bony plates | 3 |
| 3a. | No isolated spines preceding dorsal fin | 5 |
| 3b. | Dorsal fin preceded by isolated spines | 4 |
| 4a. | Series of 2-16 well-developed isolated dorsal spines preceding dorsal fin | Family Gasterosteidae Sticklebacks |
| 4b. | Series of 24-26 very short isolated dorsal spines preceding dorsal fin; snout prolonged into tube with short terminal mouth | Family Aulorhynchidae Tubesnouts |
| 5a. | Dorsal fin entirely of soft-rays | 9 |
| 5b. | Spines present in dorsal fin | 6 |
| 6a. | Dorsal fin with both soft-rays and spines | 8 |
| 6b. | Dorsal fin entirely of spines | 7 |
| 7a. | Distance from snout to anal origin less than distance from anal origin to base of caudal fin | Family Stichaeidae Pricklebacks |
| 7b. | Distance from snout to anal origin greater than distance from anal origin to base of caudal fin | Family Pholidae Gunnels |
| 8a. | Lateral line has 20 or fewer pores | Family Psychrolutidae Fathead Sculpins |
| 8b. | Lateral line has more than 20 pores | 12 |
| 9a. | Gill membranes joined to isthmus; pelvic fins club-like | Family Zoarcidae Eelpouts |
| 9b. | Gill membranes free from isthmus | 10 |
| 10a. | Body oblong; fewer than 60 dorsal rays | Family Icosteidae Ragfish |
| 10b. | Body elongate; more than 60 dorsal rays | 11 |
| 11a. | Pelvic fin soft-rays not barbel-like, appearing as a small scale or spine; tail forked | Family Trichiuridae Cutlassfishes |
| 11b. | Pelvic fin rays as barbel-like structures; caudal fin rounded | Family Bythitidae Livebearing Brotulas |
| 12a. | Snout greatly elongated, about 2 times length of maxilla; lowermost pectoral rays free | Family Rhamphocottidae Grunt Sculpin |
| 12b. | Snout about equal to or shorter than maxilla; lowermost pectoral rays connected by membranes | Family Cottidae Sculpins |