The figure denotes the position of the middle term in the premises. The different possible positions of the middle term constitute first, second, third, and fourth figures, respectively. There are 64 possible moods. Each mood can occur with each of four figures. Thus, there are 256 distinct forms that syllogisms may assume.
Form determines validity.
However, since there are entirely too many forms that syllogisms can take, I will provide some rules to follow. A valid standard-form categorical syllogism must contain exactly three terms, each of which is used in the same sense throughout the argument. Fallacy of four terms. Fallacy of equivocation. A proposition distributes a term if it refers to all members of the class designated by that term.