John Doukas (Caesar) in the context of "Constantine X Doukas"

The Byzantine Empire was ruled by emperors of the Doukas dynasty between 1059 and 1081. There are six emperors and co-emperors of this period: the dynasty's founder, Emperor Constantine X Doukas (r. 1059–1067), his brother John Doukas, katepano and later Caesar, Romanos IV Diogenes (r. 1068–1071), Constantine's son Michael VII Doukas (r. 1071–1078), Michael's son and co-emperor Constantine Doukas, and finally Nikephoros III Botaneiates (r. 7 January 1078 – 1 April 1081), who claimed descent from the Phokas family.

Under the rule of the Doukids, Byzantium was fighting a losing battle against the Seljuk Turks, losing most of its remaining possessions in Asia Minor following the catastrophic defeat at the Battle of Manzikert in 1071 and the following civil war after the death of Romanos IV Diogenes.Byzantium also incurred substantial loss of territory in the Balkans, to the Serbs, as well as losing its final foothold in Italy, to the Normans.

Constantine Doukas or Ducas (Greek: Κωνσταντίνος Δούκας, romanized: Kōnstantīnos Doúkās; c. 1074 – 12 August 1094) was Byzantine junior emperor from 1074 to 1078, and again from 1081 to 1087. He was born to Emperor Michael VII Doukas and Empress Maria of Alania in about 1074, and elevated to junior emperor probably in the same year. He was junior emperor until 1078, when Michael VII was replaced by Nikephoros III Botaneiates. Because Constantine was not made junior emperor under Nikephoros III, his betrothal to Olympias, the daughter of Robert Guiscard, was broken, which Robert Guiscard used as a pretext to invade the Byzantine Empire. John Doukas forced Nikephoros to abdicate in favor of Alexios I Komnenos in 1081, and shortly afterwards Alexios elevated Constantine to junior emperor. Constantine married Alexios's daughter Anna Komnene, and remained junior emperor until 1087, when Alexios had a son, John II Komnenos. Constantine died in c. 1095.