Khorasan comprised the present territories of northeastern Iran, parts of Afghanistan and much of Central Asia. The province was often subdivided into four quarters. Nishapur (present-day Iran), Marv (present-day Turkmenistan), Herat and Balkh (present-day Afghanistan) were the centers, respectively, of the westernmost, northernmost, southernmost, and easternmost quarters.: In the north, Khorasan stretched as far as the Oxus, and according to some descriptions, included Transoxiana (Bukhara and Samarqand in present-day Uzbekistan). Along the north it extended westward to the Caspian coast. Early Islamic usage often regarded everywhere east of so-called Jibal or what was subsequently termed Iraq Ajami (Persian Iraq), as being included in a vast and loosely-defined region of Khorasan, which might even extend to the Indus Valley and Sindh. The boundary between these two was the region surrounding the cities of Gurgan and Qumis. In particular, the Ghaznavids, Seljuqs and Timurids divided their empires into 'Iraqi' and 'Khorasani' regions. Khorasan is believed to have been bounded in the southwest by desert and the town of Tabas, known as the Gate of Khorasan,: from which it extended eastward to the mountains of central Afghanistan. Sources from the 10th-century onwards refer to areas in the south of the Hindu Kush as the Khorasan Marches, forming a frontier region between Khorasan and Hindustan.