The aim of this study is to analyse and describe in detail one of the most important trends in architecture in the 19th and 20th centuries: the evolution leading from the closed, hermetic spaces of the early cultures and the Middle Ages to the 'open space' and transparency of the 19th and 20th/20st centuries. Historically, the focus is on the 'diaphanous' space of the Gothic cathedral, the opening of the late-Baroque dome towards the sky, the transparency of exhibition halls and hothouses in the 19th century, and the glass dreams of the early 20th century. The steel-and-glass technology of the past one hundred years has permitted even more transparency, openness, and dematerialisation on a scale never seen before.