Sigma Sigma Sigma is a member of the National Panhellenic Conference (NPC), an umbrella organization encompassing 26 national sororities or women's fraternities, which focus on service, education, scholarship programming and social activities. Once a sorority exclusively for teacher's/educational colleges, Tri Sigma became a full NPC member in 1951 and, as a social sorority, now admits members without limits based on major. The sorority counts a membership of more than 125,000 women, hosts chapters on more than 112 college campuses, and maintains over 90 alumnae chapters. The sorority's own headquarters are located in Woodstock, Virginia.