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Fraternities and sororities are social organizations at colleges and universities. A form of the social fraternity, they are prominent in the United States, Canada, and the Philippines, with smaller numbers existing in France, the United Kingdom, and elsewhere. Similar organizations exist in other countries as well, including the Studentenverbindungen of German-speaking countries.

Similar, but much less common, organizations also exist for secondary school students. In modern usage, the term "Greek letter organization" ("GLO") is often synonymous with the terms "fraternity" and "sorority". Two additional types of fraternities, professional fraternities and honor societies, incorporate some limited elements of traditional fraternity organization but are generally considered a different type of association. Traditional fraternities of the type described in this article are often called "social fraternities".

Generally, membership in a fraternity or sorority is obtained while an undergraduate student but continues, thereafter, for life. Some of these organizations can accept graduate students as well as undergraduates, per constitutional provisions.

While individual fraternities and sororities vary in exact organization and purpose, most share five common elements:

Fraternities and sororities sometimes engage in philanthropic activities; often host parties; provide "finishing" training for new members, such as instruction on etiquette, dress, and manners; and create networking opportunities for their newly graduated members.

The first fraternity in North America to incorporate most of the elements of modern fraternities was Phi Beta Kappa, founded at the College of William and Mary in 1775. The founding of Phi Beta Kappa followed the earlier establishment of two other secret student societies that had existed at that campus as early as 1750. In 1779 Phi Beta Kappa expanded to include chapters at Harvard and Yale. By the early 19th century, the organization transformed itself into a scholastic honor society and abandoned secrecy. In 1825 Kappa Alpha Society, the oldest extant fraternity to retain its social characteristic, was established at Union College. In 1827, Sigma Phi and Delta Phi were also founded at the same institution. The foundation of this triad established Union College as the Mother of Fraternities.

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