Although the Constitution of the United States is a single written document, American constitutional law – the subject of this book – is a complex social, cultural, and political practice that includes much more than the written Constitution. Courts, especially the Supreme Court, interpret the Constitution. So do legislators and other government officials as they consider their responsibilities. Very commonly, however, “interpretation” of the Constitution depends on a variety of considerations external to the text. These include the historical practices of Congress and the President, previous judicial decisions or “precedents,” public expectations, practical considerations, and moral and political values. By talking about constitutional law as a “practice,” I mean to signal that factors such as these are elements of the process from which constitutional law emerges.

Strikingly, arguments about how to interpret the Constitution occur frequently in constitutional practice – not least among Justices of the Supreme Court. (Among the difficulties in studying constitutional law is that the rules of constitutional interpretation are nowhere written down in authoritative form, and any one person's attempt to formulate them would trigger dispute.) Nonetheless, a few fixed points command nearly universal agreement. First, at the center of the frequently argumentative practice of constitutional law stands the written Constitution of the United States. Second, when the Supreme Court decides a case, its ruling binds public officials as well as citizens, despite their possibly contrary views. Supreme Court rulings occasionally encounter resistance. In a few instances they have provoked actual or threatened defiance – matters that I discuss later in this book. Normally, however, the Court gets to say authoritatively what the Constitution means.