Union County is the northeasternmost county in the U.S. state of New Mexico. As of the 2020 census, the population was 4,079, making it the fourth-least populous county in New Mexico. Its county seat is Clayton. The county was formed in 1894. Union County borders Colorado to the north, and Oklahoma and Texas to the east.

Quay County is a county in the state of New Mexico. As of the 2020 U.S. census, the population was 8,746. Its county seat is Tucumcari. The county was named for Pennsylvania senator Matthew Quay, who supported statehood for New Mexico. Its eastern border is the Texas state line, approximately 103.04 degrees west longitude.