Primary Interstate Highways are the major interstate highways of the United States and have a one or two-digit route number. "Even" (0, 2, 4, 6, or 8) route numbers are given to east/west routes, with the smaller numbered routes in the south (I-10) and bigger numbered routes in the north (I-90). Similarly, "odd" (1, 3, 5, 7, or 9) route numbers are given to north/south routes, with the smaller numbered routes in the west (I-5) and the bigger numbered routes in the east (I-95).