Primary Interstate Highways of the United States are numbered with one- or two-digit designations. Their associated auxiliary highways have three-digit numbers. Generally, odd-numbered routes run north-south, with lower numbers in the west and higher numbers in the east; even-numbered routes run east-west, with lower numbers in the south and higher numbers in the north. Routes divisible by "5" (for example, I-40 or I-75) are major routes, often running coast-to-coast or border-to-border.