This book “follows the decisions” and studies how in different countries amendment rules have a fundamental impact on how easy it will be to make amendments and how frequent and/or significant these amendments are likely to be. The amendment rules specify how many institutions are required to approve the change, what the conditions are (quorums, qualified majorities) in each one, and whether these rules operate in a conjunctive or disjunctive way. All these conditions have specific effects on the constitutional rigidity of a country, which in turn is expected to have effects in three different directions. First, the frequency and significance of amendments are inversely related with the constitutional rigidity of the country and its variance. The more significant the amendments, the stronger this relationship. Second, the length of the constitution is correlated with a series of negative results like time inconsistency as well as economic variables. Third, constitutional rigidity affects judicial independence of the supreme or constitutional court and its variance. The main contribution of this book is the combination of these ideas in a coherent framework, from theory to case studies or application to all democracies.