This article argues that a better trace of the operation of a balance of power system can be obtained if two dimensions, alliances and capabilities, are combined into a single measure of bloc concentration. After reviewing the basic features of a balance of power system to show why this is the case, such an index is constructed. Several patterns of bloc concentration consistent with the operation of a balance of power system are derived and examined, using data for the European major powers from 1824 through 1914, and for all the major powers from 1919 through 1965. The findings are consistent with the existence of a balance of power system in the earlier era and with a form of balancing behavior in the post-World War I era.