Frank N. Blanchard created a key for identifying North American snakes, motivated by the absence of a comprehensive reference on the subject. The key covers all species and subspecies north of the Rio Grande, including Lower California, aligning with Stejneger and Barbour's Check List of North American Amphibians and Reptiles. His goals were to enable quick and accurate identification without dissection, provide a synopsis of genera and species, and accurately map their distributions. The key is based largely on Blanchard's examination of specimens, mainly from the United States National Museum. He borrowed classifications for certain genera from other experts and adapted existing studies for Thamnophis and Pituophis. The author used practical systematic standards, considering user convenience, and included a selected list of references for further information.