  Another innovation in the syllogistic is Aristotle’s use of letters in place of terms. The letters may originally have served simply as abbreviations for terms (e.g. An. Post. A 13); but in the syllogistic they seem mostly to have the function either of schematic term letters or of term variables with universal quantifiers assumed but not stated. Where he uses letters, Aristotle tends to express the four types of categorical sentences in the following way (with common later abbreviations in parentheses):