This introduction presents an overview of the key concepts discussed in the subsequent chapters of this book. The book addresses the first phase of open architecture. This phase can be regarded as the modern era of architecture. The book considers the spirit of the era underlying the first phase of open architecture, as well as the efforts that helped bring the phase to realization. It also addresses the concepts of “Vernacular” and “System.” The book discusses the achievements and practices of Ryo Kenmochi and Katsuhiko Ohno in Japan. It focuses on the importance of understanding the role of salespeople, who worked to form relationships with inhabitants, as well as the tatami module, which is unique to Japanese architecture. The book also discusses the clear example of the housing industry and describes actual cases after the third phase of open architecture to address the role of the new type of experts in further developing opportunities for people’s independence.