From executive and legislative behavior to the boundaries of the state's monopoly on violence, "bright lines'' --- informal norms of acceptable behavior --- play a critical role in regulating political interactions, especially in the gray areas left by formal law. While previous research has analyzed the influence of informal rules on politics, the question of how such norms emerge remains largely unexamined. This paper presents a tractable accountability model that explains the emergence and impact of political bright lines. The theory shows how crossing bright lines provides citizens and politicians with informational signals about a leader's quality and potential autocratic preferences. However, the stability and efficiency of these norms vary: highly restrictive lines are often uninformative and short-lived, while overly loose lines are long-lived but inefficient. Bright lines of intermediate restrictiveness are most likely to persist. The paper also examines the strategic establishment of bright lines in constitutional design. The findings hold implications for understanding the interplay between formal and informal institutions, the dynamics of institutional development and stability, and the risks of democratic backsliding.