Constitutional design proceeds under the assumption that institutions have predictable consequences, but modern political science has not pursued the empirical verification of these predicted consequences with much vigor. I shall attempt to link the theoretical premises underlying one important aspect of constitutional design, the amendment process, with the empirical patterns revealed by a systematic, comparative study of constitutions. An examination of all amendments in the 50 American states since 1776 reveals patterns that are then confirmed using data from 32 national constitutions. The interaction of the two key variables affecting amendment rate can be described by an equation that generates predicted amendment rates close to those found in the cross-national empirical analysis. A constitution's length measured in number of words, the difficulty of an amendment process, and the rate of amendment turn out to have interlocking consequences that illuminate principles of constitutional design.