The search for the precise locus of res judicata within procedural structure is more than a taxonomic exercise—it is a test of how a legal system balances finality and fairness. Traditional doctrinal dichotomies, such as those between claim and cause of action or between operative part and reasoning, have long shaped the debate but often obscure the true boundaries of what has been adjudicated. This chapter contends that the “object of res judicata” is best understood as the subject matter in dispute, whether confined to the main issue or extended to prejudicial determinations. By shifting the focus to the material legal relationship resolved, rather than its procedural framing, the analysis challenges the restrictive “triple identity” criterion and proposes a more coherent and constitutionally grounded distribution of judgement effects. Drawing on Brazilian legislative reform and comparative law, the chapter confronts the persistent gap between formal claim structures and substantive dispute resolution. For both proceduralists and legal theorists, the implications are provocative: perhaps the law has been searching for res judicata in the wrong place. Could greater stability—and the peace it promises—be achieved by embracing certain prejudicial determinations within the scope of finality? This chapter not only raises the question but lays the groundwork for rethinking the architecture of civil adjudication itself.