There is some uncertainty for the exact relationship between titles marzbān, spāhbed, kanārang, pāygōsbān (Parthian ptykwspn, Sasanian paygospān or padhospān) and ostāndār. The historical sources blur the distinction between the marzbān and spāhbed (army general or military governor), implying marzbān was a military title strictly limited to the frontier marches and provinces. The least clear is the distinction with kanārang, apparently an East-Iranian derivation of marzbān in the province Abarshahr in Central Asia. The pāygōsbān, meaning "guardian of the district", is an uncertain title, seemingly provincial military commanders or governors, while the marzbān meant "guardian of the borders, provinces". Perhaps the pāygōsbān lacked civilian duties. The ostāndār was the governor of an ostān (province or district within a province). The primary sources imply the marzbān was a provincional function practiced for a single or multiple provinces, but there is no evidence for a "quarter of the empire", as al-Masudi entitled Šahrwarāz (629 AD). The rank of marzbān, like most imperial administration, was mostly patrimonial, and was passed down through a single family for generations. The marzbāns of greatest seniority were permitted a silver throne, while marzbāns of the most strategic border provinces, such as the province of Armenia, were allowed a golden throne. In military campaigns the regional marzbāns could be regarded as field marshals, while lesser spāhbeds could command a field army. The function of marzbān changed over the years, with smaller territorial units being part of the civil administration. In the early years the main marzbān regions were Armenia, Beth Aramaye, Pars, Kirman, Spahan, Adurbadagan, Tabaristan, Nishapur, Tus, Sakastan, Mazun, Harev, Marv and Sarakhs, several mentioned belonging to the Greater Khorasan. Some regions enjoyed considerable autonomy while other were militarily more important, for example the Adurbadagan facing the Caucasus was special military frontier. Marzbāns were granted the administration of the border provinces and were responsible for maintaining the security of the trade routes, fighting the encroaching nomadic tribes such as Bedouin Arabs, White Huns and Oghuz Turks, and holding the first line of defense against settled enemies such as Romans and Kushans. During the reign of Khosrow I (531–579 AD) were held military reforms by which were created four frontier regions (Khwarasan, Khwarwaran, Nemroz, Adurbadagan) with spāhbed in charge, sometimes still called as marzbān, but now generally considered for more central provinces. Also, the previous gentry rank dihqānān was moulded into influential "nobility of service" which became the