Baird's Manual of American College Fraternities (1879)/Class fraternities
In addition to the regular fraternities, there are in the Eastern colleges certain societies which are the especial property of one class: Thus we have senior, junior, sophomore, and freshman societies. Many of them have Greek names and badges, and some few have several chapters. They interfere in no way with the fraternities, and usually serve to bring the leading spirits of each class together. In the following account of the class societies, I am indebted to a work entitled "Four Years at Yale," published anonymously in 1871, for information in regard to the Yale societies. The first class society was probably Phi Beta Kappa, which at first only admitted seniors. It was followed by Chi Delta Theta and others at Yale, and the idea has been extensively copied. By an “open” society is meant one which admits members whether they belong to other fraternities or not.