The syllogism consists of three categorical statements (two parcels and one conclusion which to standard record is written below the line). The subject of the conclusion is designated (usually) by a letter S, and a predicate - P, but in a syllogism of S is called as the smaller term, and P - big; both of them are called as extreme terms. The term which is twice repeating in parcels is called as an average (Latin - terminus medius) and is designated by a letter M.
Analyzing this example, we the term taking the subject's place recognizes that, is distributed in the general statements (And, and the term taking a predicate place is distributed in negative statements (, About). Strict following to this definition is a basis of the so-called narrow theory of a syllogism.
The rule is sometimes formulated: "In a syllogism has to be three and only three terms.". The instruction on this requirement is directed on avoiding a mistake which is called as quadrupling of terms (it is based on conscious or unconscious use of the phenomenon of a homonymy).
If to take quantitative and qualitative characteristics of the parcels entering a syllogism and the conclusions into account, we will receive the versions called by modes. The mode registers three letters (from A, E, I, O) in such sequence - a big parcel, a smaller parcel, the conclusion.