well Phi Beta Kappa, which is generally considered the first fraternity has three. But four of the first five social orgs that are still around have only two letters.

According to Wikipedia, according to Baird's lol: In 1867, a society called I. C. Sorosis was founded as a women's fraternity and, while it closely modeled men's fraternities, the membership was not confined to college students. As Chi Theta Delta had disbanded and the Adelphean and Philomathean societies had not yet taken Greek letter names, Kappa Alpha Theta gains distinction for the first Greek letter women's fraternity to be founded with continued existence. It was founded in January 1870 at DePauw University. Kappa Kappa Gamma followed suit when it was established October 1870 at Monmouth College.

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