In 1966, Yale began discussions with its sister school Vassar College about merging to foster coeducation at the undergraduate level. Vassar, then all-female and part of the Seven Sisters—elite higher education schools that historically served as sister institutions to the Ivy League when the Ivy League still only admitted men—tentatively accepted, but then declined the invitation. Both schools introduced coeducation independently in 1969. Amy Solomon was the first woman to register as a Yale undergraduate; she was also the first woman at Yale to join an undergraduate society, St. Anthony Hall. The undergraduate class of 1973 was the first class to have women starting from freshman year; at the time, all undergraduate women were housed in Vanderbilt Hall at the south end of Old Campus.[citation needed]

What sister school of Yale's considered merger in 1966?