South Dakota's state highways were assigned in a numbering pattern that followed that of the U.S. Highways followed upon their inception. East-west highways carried even numbers and increased from north to south – while north-south highways carried odd numbers and increased from east to west. This holds true only for 2-digit highways. 3-digit highways follow the odd-even routing, but do not sequentially remain near a "parent" route as a spur or alternate route, instead being more independent of any parent 2d route.