Eisenstein's contribution to the development of cinema rested primarily in his theory of editing, or montage, which focused on the collision of opposites in order to create a new entity. One of the greatest achievements in editing is the Odessa Steps sequence, in his film Potemkin (1925). Eisenstein intercut between shots of townspeople trapped on the steps by Czarist troops, and shots of the troops firing down upon the crowd. Members of the crowd became individual characters to viewers as the montage continued. Within the editing track the fate of these individuals was played out. A mother picks up her dead child and confronts the troops. Then she is shot. A student looks on in terror and then flees--his fate uncertain. An old woman prays to be spared, but she is killed by a soldier who slashes her face with his saber. When a woman holding her baby carriage is killed, she falls to the steps, and the carriage begins a precipitous decline--shots of the baby crying are intercut with wide shots of the carriage rolling down the steps. To Eisenstein, each individual shot contributed an energy within the editing track that yielded far more than the sum total of shots. In other words, the "combination" of shots through editing created a new entity, based on the expressive emotional energy unleashed through the editing process