A constitutional monarchy keeps a hereditary head of state while binding that office to a written or unwritten constitution, so the crown reigns without ruling. The arrangement rarely arrives on one day: it is usually the product of a slow transfer of real power away from a monarch who once governed personally, toward an elected legislature and a cabinet that answers to it.
The United Kingdom is the clearest example of that slow transfer. The Glorious Revolution of 1688 to 1689 and the English Bill of Rights it produced first subordinated the crown to Parliament, ending the monarch's claim to rule by unchecked prerogative. Executive power then migrated to ministers who needed the confidence of the House of Commons, a practice that hardened into convention over the following two centuries rather than being written down in any single founding document.
Other constitutional monarchies were built more deliberately. Japan's Meiji Constitution of 1889 created a monarchy bound by law at the government's initiative, part of a wider modernization program, and its postwar successor pushed the emperor's role further toward a purely ceremonial one. Both paths, gradual English convention and deliberate Japanese codification, arrive at the same structure: a monarch who reigns as a symbol of continuity while an elected government exercises the power of the state.