Despite the importance of supermajority rules in protecting minorities, our understanding of supermajority rules has been limited to the experience of the U.S. Senate. This study seeks to contribute to our understanding of institutional choice by introducing another case of supermajority-rule adoption. Once known for legislative brawls, the National Assembly of Korea reformed its procedures in 2012 to require a supermajority to pass controversial legislation. The evidence presented in this study suggests that supermajority rules were adopted in an attempt to reduce chaos and uncertainty in the lawmaking process by shifting the legislature from a norm-based one to a rule-based one. This article demonstrates this by drawing on official documents, interviews, and an automated text analysis of newspapers.