court appointment, as promising young men had the opportunity to catch the Emperor's attention. Officers in the tagmata came primarily either from the relatively well-off urban aristocracy and officialdom, or the landed aristocracy of the Anatolian themes, which increasingly came to control the higher military offices of the state. Nevertheless, the tagmata, as indeed military and state service in general, offered a degree of upwards social mobility for the lower strata of society. In their heyday in the 9th and early 10th centuries, there were four tagmata proper ("τὰ δʹ τάγματα"): the Scholai (Gr. Σχολαί, "the Schools"), were the most senior unit, the direct successor of the imperial guards established by Constantine the Great (r. 306–337). The term scholarioi (σχολάριοι), although in its stricter sense referring solely to the men of the Scholai, was also used as a general reference for all common soldiers of the tagmata. the Exkoubitoi or Exkoubitores (Lat. Excubiti, Gr. , "the Sentinels"), established by Leo I. the Arithmos (Gr. , "Number") or Vigla (Gr. Βίγλα, from the Latin word for "Watch"), promoted from thematic troops by the Empress Eirene in the 780s, but of far older ancestry, as the archaic names of its ranks indicate. The regiment performed special duties on campaign, including guarding the imperial camp, relaying the Emperor's orders, and guarding prisoners of war. the Hikanatoi (Gr. , "the Able Ones"), established by Emperor Nikephoros I (r. 802–811) in 810. Other units closely related to the tagmata, and often included among them, were: the Noumeroi (Gr. Νούμεροι, from the Latin numerus, "number") were a garrison unit for Constantinople, which probably included the Teichistai (Gr. Τειχισταί) or tōn Teicheōn regiment (Gr. , "of the Walls"), manning the Walls of Constantinople. The unit's origins may lie as far back as the 4th–5th centuries. the Optimatoi (Gr. , from Latin optimates, "the best"), although formerly an elite fighting unit, had by the 8th century been reduced to a support unit, responsible for the mules of the army's baggage train (the τοῦλδον, touldon). Unlike the tagmata, it was garrisoned outside Constantinople and closely associated with its garrison area: the thema Optimatōn, which lay across Constantinople and comprised northern Bithynia. The commanding domestikos of the Optimatoi was also the governor of the thema. the men of the central Imperial Fleet (βασιλικόν πλώιμον, basilikon plōimon), are also counted among the tagmata in some sources. the Immortals (Gr. ), were