The title derives from the Greek words meros (Greek: μέρος, "part, division") and archein (ἄρχειν, "to rule, command"). The term merarchēs is attested for the first time in the late 6th century in the Stratēgikon, a military manual attributed to the Byzantine emperor Maurice (r. 582–602), although the historian Warren Treadgold has suggested that the rank and the corresponding formation date back to the reign of Emperor Zeno (r. 474–499). In the time of the Stratēgikon, a field army (commanded by a stratēgos) comprised usually three merē, each probably some five to seven thousand-strong. The meros in turn was divided into several moirai consisting of a number of tagmata or banda, each commanded by a doux.
This division was maintained in the later Byzantine army, although already from the 7th century, the term merarchēs became used less frequently, being dropped in favour of tourmarchēs; likewise, the tourma replaced the meros both in technical and common parlance. The equivalence of the two terms is explicitly attested in the Taktika of Emperor Leo VI the Wise (r. 886–912). The tourmai were now the major territorial and tactical subdivisions of a provincial army corps (thema). Each thema, again under a stratēgos, was normally divided into three tourmai, which in turn were further divided into droungoi (analogous to the older moirai) and then banda. Depending on the size of the thema, the number of the banda varied, and consequently the numerical strength for each meros/tourma could range from circa 1,000 to 5,000 men.
Since the merarchēs – also found in the corrupted form meriarchēs (Greek: μεριάρχης) – is sometimes distinguished in the sources (e.g. the Klētorologion of Philotheos) from the other tourmarchai, the scholar John B. Bury suggested that in the 9th and 10th centuries, the merarchēs was a distinct post, held by the tourmarchēs attached as an aide and deputy to the thematic stratēgos with no geographical area under his command, as opposed to the two "regular" tourmarchai. The discovery of a seal of a merarchēs of Knossos shows that they did hold territorial assignments, leading Alexander Kazhdan to reject Bury's hypothesis in the Oxford Dictionary of Byzantium. Military historian John Haldon, in his edition of the Three Treatises on Imperial Military Expeditions, in essence agreed with Bury's proposition, regarding the merarchēs as the commander of the tourma comprising the district where the thematic headquarters were located. According to Haldon, this would also explain his apparently lower rank relatively to the other tourmarchai, since he was a member of the stratēgos's staff and not an independent commander.
- Bury, John B. (1911). The Imperial Administrative System of the Ninth Century: With a Revised Text of the Kletorologion of Philotheos. London, United Kingdom: Oxford University Press.
- Haldon, John F. (1990). Constantine Porphyrogenitus: Three Treatises on Imperial Military Expeditions. Vienna, Austria: Verlag der Österreichischen Akademie der Wissenschaften.
- Kazhdan, Alexander Petrovich, ed. (1991). The Oxford Dictionary of Byzantium. New York, New York and Oxford, United Kingdom: Oxford University Press. ISBN 978-0-19-504652-6.
- Treadgold, Warren T. (1995). Byzantium and Its Army, 284–1081. Stanford, California: Stanford University Press. ISBN 0-8047-3163-2.