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Porphyrogénnētos, Latinized as Porphyrogenitus (Greek: Πορφυρογέννητος, literally "born in the purple") was an honorific title given to a son, or daughter (Πορφυρογέννητη, Porphyrogénnētē, Latinized Porphyrogenita), of a reigning emperor in the Byzantine Empire. However, not every imperial prince or princess was accorded this distinction. There was a prescribed set of circumstances that had to be in place before this title could be granted.
Throughout their history, the Byzantine emperors never restricted the imperial succession to primogeniture. The concept of dynastic legitimacy however was inherited from earlier monarchies, and reached its peak in the period when the Macedonian dynasty, Komnenoi and Palaiologoi dominated the imperial throne. The title porphyrogénnētos is not attested before 846, but as a declaration of legitimacy it was found useful in dynastic nomenclature in the 10th century, and survived until well into the Palaiologan period.
The Porphýra/Porphyry Chamber 
This designation's most distinctive condition was that the child be born in the "Πορφύρα" (Porphýra, the Purple or Porphyry Chamber, a pavilion of the Great Palace of Constantinople): no child born anywhere else could legitimately be called Porphyrogénnētos. This pavilion was a free-standing building in the Great Palace complex in Constantinople. As the Porphyrogennētē Anna Komnena described it, the room rested on one of the Palace's many terraces, overlooking the Sea of Marmora and the Bosphorus Strait, "where the stone oxen and the lions stand" (i.e. the Boukoleon Palace), and was in the form of a perfect square from floor to ceiling, with the latter ending in a pyramid. Its walls, floor and ceiling were completely veneered with Imperial Porphyry, which was "generally of a purple colour throughout, but with white spots like sand sprinkled over it."
Basileus and Augusta 
The other important qualification for status as a Porphyrogénnētos was that the father must be a reigning Basileus (Greek: βασιλεύς, the Greek word for King or Emperor), and the mother must be married to the Basileus (and therefore an empress) and additionally must have undergone a formal, sacred ceremony creating her an Augusta.
Byzantine emperors - Porphyrogénnētoi or not - were conventionally viewed as personages legitimated by God as rulers; the focus of the title Porphyrogénnētos was that it imbued its honoree with the sense of the pre-ordained, of earthly continuity and legitimacy.
Some Imperial diplomatic missions were accomplished on the condition of a porphyrogénnēta bride being sent to solidify the bargain, or of a foreign princess coming to Constantinople to marry a porphyrogénnētos.