Details

The letters A, E, I, and O have been used since the medieval schools to form mnemonic names for the forms as follows: "Barbara" stands for AAA, "Celarent" for EAE, and so on [4].

References

[1] L. Carroll, Symbolic Logic and the Game of Logic, New York: Dover Publications, 1958.

[2] I. M. Copi and C. Cohen, Introduction to Logic, 9th ed., New York: Macmillan Publications, 1994 pp. 214–218.

[3] J. M. Bocheński, A History of Formal Logic, 2nd ed. (I. Thoma, trans. and ed.), New York: Chelsea Publishing Company, 1970 p. 235.

[4] Wikipedia. "Syllogism." en.wikipedia.org/wiki/Syllogism.