Theta Xi Fraternity (ΘΞ) is a North American Greek-letter social college fraternity. It was founded at Rensselaer Polytechnic Institute (RPI) on April 29, 1864. Of all the social fraternities today, Theta Xi was the only one founded during the Civil War. Its Grand Lodge is headquartered in suburban St. Louis, Missouri. Since its inception, Theta Xi has grown to include more than 60,000 initiated members. Currently, there are approximately 50 active chapters.