What snake family includes vipers, adders, and rattlesnakes?

The snake family that includes vipers, adders, and rattlesnakes is Viperidae.

Viperidae contains the true vipers and pit vipers, all of which are venomous. Its members occur across much of the Americas, Africa, Europe, and Asia. Rattlesnakes are American pit vipers, while many familiar Old World adders and vipers belong to the subfamily Viperinae.

A key viperid feature is a pair of long, hollow, hinged fangs. These fangs fold against the roof of the mouth when closed and swing forward during a strike, allowing venom to be delivered deeply. Pit vipers, including rattlesnakes, also have heat-sensing pits between the eyes and nostrils.

The family is easy to mix up with Elapidae, which includes cobras and coral snakes. Elapids generally have fixed front fangs, whereas viperids have rotating fangs. “Adder” is a common name rather than a separate family: the common adder of Europe, for example, is Vipera berus, a member of Viperidae.