| 1a. | Caudal and dorsal fins broadly rounded | Family Cyprinodontidae Killifishes / Pupfishes |
| 1b. | Caudal fin forked | 2 |
| 2a. | Tail heterocercal with upper lobe extended; 5 (rarely 7) rows of bony shields on sides and back | Family Acipenseridae Sturgeons |
| 2b. | Caudal lobes equal or lower lobe elongated; no bony shields on body | 3 |
| 3a. | Dorsal fin attached posteriorly, much closer to tail than to head; dorsal and anal fins followed by 5-7 finlets | Family Scomberesocidae Sauries |
| 3b. | Dorsal fin about midbody; dorsal fin origin well in advance of anal fin origin; dorsal and anal fins not followed by finlets | 4 |
| 4a. | Lateral line present; serrate spine in anterior portion of dorsal and anal fins | Family Cyprinidae Carps and Minnows |
| 4b. | Lateral line absent; no serrate spine in dorsal and anal fins | 5 |
| 5a. | Jaws terminal, even, or lower jaw projecting slightly beyond upper jaw | Family Clupeidae Herrings |
| 5b. | Snout overhanging lower jaw | Family Engraulidae Anchovies |