If you follow the traditional rule, you say that the whole comprises the parts and that the parts compose the whole. Thus you would say The Union comprises fifty states and Fifty states compose (or constitute or make up) the Union. While writers often maintain this distinction, comprise is increasingly used in place of compose, especially in the passive: The Union is comprised of fifty states. Don’t be surprised if this usage still elicits comments, however. In an earlier survey, a majority of the Usage Panel found this use of comprise unacceptable.
