ATΩ (Alpha Tau Omega) (commonly known as ATO, Taus, Alpha Taus) is an American social fraternity that annually ranks among the top ten national fraternities for numbers of chapters and total number of members. ATO has more than 250 active and inactive chapters with more than 196,000 members and more than 6,500 undergraduate members. ATO is also one-third of the Lexington Triad, along with Kappa Alpha Order and Sigma Nu. The oldest active chapter is the Delta Chapter located at the University of Virginia.