The title of megas domestikos (Greek: μέγας δομέστικος), the Grand Domestic in English, was given in the 11th–15th centuries to the commander-in-chief of the Byzantine land army, below the Emperor. Its exact origin is somewhat unclear: it is first mentioned in the 9th century, and most likely derives from the older office of domestikos tōn scholōn ("Domestic of the Schools"), with the epithet megas added to connote the supreme authority of its holder, following contemporary practice evident in other offices as well. Both titles appear to have co-existed for a time, with the megas domestikos being a more exalted variant of the "plain" Domestics of the East and the West, until the late 11th century, when it became a separate office and replaced the "plain" Domestics as commander-in-chief. Nevertheless, the office was still sometimes referred to as the "Grand Domestic of the Schools" or "of the army", creating some confusion as to its exact identity. In the Komnenian period, in an echo of the 10th-century arrangements, the megas domestikos would sometimes command the entire field army of East or West.
Following the Fourth Crusade, it appears that in the Latin Empire and the other Latin states formed on Byzantine soil, the title of megas domestikos was used as the Greek equivalent of the Western title of [grand] seneschal (Latin: [magnus] senescallus). In the Palaiologan period, the office initially fell in rank below those of prōtovestiarios and the megas stratopedarchēs, but was raised by the mid-14th century to be one of the highest ranks, directly below that of Caesar. It remained the formal head of the army, although in fact it was bestowed to generals and high-ranking courtiers alike, among others George Mouzalon, John Palaiologos (brother of Michael VIII), Michael Tarchaneiotes, Alexios Strategopoulos and John Kantakouzenos (the future John VI). The office also included various ceremonial functions, as detailed in the account of offices of pseudo-Kodinos.
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