Alaska Airlines is a major United States airline headquartered in SeaTac, Washington, within the Seattle metropolitan area of the state of Washington. It is the fifth-largest airline in the United States when measured by fleet size, scheduled passengers carried, and number of destinations served. Alaska, together with its regional partners, operates a large domestic route network, primarily focused on connecting cities on the West Coast of the United States to over one hundred destinations in the contiguous United States, Alaska, Hawaii, Canada, Costa Rica, and Mexico. Alaska Airlines is not a member of any of the three major airline alliances. However, it has codeshare agreements with 17 airlines, including member airlines of oneworld, SkyTeam, Star Alliance, and unaffiliated airlines. Regional service is operated by sister airline Horizon Air and independent carrier SkyWest Airlines.