Romeo and Juliet begins with a Chorus, which establishes the plot and tone of the play. This device was hardly new to Shakespeare, and in fact mirrors the structure of Arthur Brooke's The Tragical History of Romeus and Juliet, from which Shakespeare adapted Romeo and Juliet.
Additionally, the Chorus poses the question of whether or not Romeo and Juliet is a tragedy. During Shakespeare's time, it was typical for a tragedy to begin with a Chorus. In Romeo and Juliet, the opening sonnet presents dire enough circumstances to support that convention. However, tragedy in its strictest form presupposes certain formal conceits. Most important is the idea that an individual (or individuals) is (or are) defeated by forces beyond his or her control; tragedies most often celebrate human willpower in the face of bad luck or divine antagonism. And yet, the forces at play in Romeo and Juliet are hardly beyond human control. Instead, the Montagues and Capulets have allowed their feud to fester. This is evident from the first scene, when even the patriarchs of both families enter the public street fight, ready to kill. The Chorus introduces Shakespeare's unique approach to tragedy by introducing certain established tropes of that genre but by refusing to lay the blame at the universe’s feet.
In addition, the Chorus also introduces certain sources of dramatic tension that re-appear throughout the rest of the play. For example, the diametric opposition between order and disorder is central to to Romeo and Juliet. In the Prologue, the Chorus speaks in sonnet form, which was usually reserved for a lover addressing his beloved. The sonnet is a very structured form of poetry, which indicates a level of order.
