The harem system first became fully institutionalized in the Islamic world under the Abbasid caliphate. Seclusion of women was established in various communities of the Mediterranean, Mesopotamia, and Persia before the advent of Islam, and some scholars believe that Muslims adopted the custom from the Byzantine Empire and Persia, retrospectively interpreting the Quran to justify it. Although the term "harem" does not denote women's quarters in the Quran, a number of Quranic verses discussing modesty and seclusion were held up by Quranic commentators as religious rationale for the separation of women from men, including the so-called "hijab verse" (33:53). In modern usage "hijab" colloquially refers to the religious attire worn by Muslim women, but in this verse it meant "veil" or "curtain" that physically separates female from male space. Although classical commentators agreed that the verse spoke about a curtain separating the living quarters of Muhammad's wives from visitors to his house, they usually viewed this practice as providing a model for all Muslim women.