The advantage in three acts is that each allows a longer space than does the division into four or five acts in which characterization may develop before the eyes of the audience, or a larger number of illustrative actions bearing on the central purpose of the act may be shown. The offset is that three acts provide only two breaks by which the passing of time may be suggested. Neither four nor three acts have any essential superiority over each other, or over five acts. Five acts, in and of themselves, have no superiority over four or three; nor, as some persons have seemed to think, are they the only divisions in which a drama in verse may be written. Avoidance of awkward changes of scene within an act may compel use of four or five acts rather than three. The more episodes in the story to be dramatized, the more aspects of character to be shown by action, the more acts or scenes the dramatist must use. If long spaces of time must be allowed for because they are part of the story or marked changes of character demand them, the dramatist will need more _entr'acte_ space, and, consequently, more acts. It is, then, necessary change of place and passage of time which are the chief factors in determining choice among three, four, or five acts.
For centuries theoretical students of the drama have worried themselves about the two unities: place and time. Practising dramatists, however, have usually found that generalizations in regard to them help little and that in each individual play they must work out the place and time problems for themselves. Practice as to shifting scenes has depended most, and always will, upon whether the physical conditions of the stage permit many real or imagined shifts. The Greek stage, with its fixed background and its chorus nearly always present, forced an attempt at unity of place, though the Greeks often broke through it.
Unity of action was the first dramatic law of the ancients; unity of time and place were mere consequences of the former which they would scarcely have observed more strictly than exigency required had not the combination with the chorus arisen. For since their actions required the presence of a large body of people and this concourse always remained the same, who could go no farther from their dwellings nor remain absent longer than it is customary to do from mere curiosity, they were almost obliged to make the scene of the action one and the same spot and confine the time to one and the same day. They submitted bona fide to this restriction; but with a suppleness of understanding such that in seven cases out of nine they gained more than they lost thereby. For they used this restriction as a reason of simplifying the action and to cut away all that was superfluous, and thus, reduced to essentials, it became only the ideal of an action which was developed most felicitously in this form which required the least addition from circumstances of time and place.