Houston:

Houston (i/ˈhjuːstən/ HYOO-stən) is the most populous city in Texas and the fourth most populous city in the United States, located in Southeast Texas near the Gulf of Mexico. With a census-estimated 2014 population of 2.239 million people, within a land area of 599.6 square miles (1,553 km2), it also is the largest city in the Southern United States, as well as the seat of Harris County. It is the principal city of Houston–The Woodlands–Sugar Land, which is the fifth most populated metropolitan area in the United States.

Please answer a question about this article. If the question is unanswerable, say "unanswerable". What is Houston the non-principle city of?
unanswerable