Taxes levied on local populations in the wake of early Islamic conquests could be of three types, based on whether they were levied on individuals, on the land, or as collective tribute. During the first century of Islamic expansion, the words "jizya" and "kharaj" were used in all these three senses, with context distinguishing between individual and land taxes ("kharaj on the head," "jizya on land," and vice versa). In the words of Dennett, "since we are talking in terms of history, not in terms of philology, the problem is not what the taxes were called, but what we know they were." Regional variations in taxation at first reflected the diversity of previous systems. The Sasanian Empire had a general tax on land and a poll tax having several rates based on wealth, with an exemption for aristocracy. In Iraq, which was conquered mainly by force, Arabs controlled taxation through local administrators, keeping the graded poll tax, and likely increasing its rates to 1, 2 and 4 dinars. The aristocracy exemption was assumed by the new Arab-Muslim elite and shared by local aristocracy by means of conversion. The nature of Byzantine taxation remains partly unclear, but it appears to have involved taxes computed in proportion to agricultural production or number of working inhabitants in population centers. In Syria and upper Mesopotamia, which largely surrendered under treaties, taxes were calculated in proportion to the number of inhabitants at a fixed rate (generally 1 dinar per head). They were levied as collective tribute in population centers which preserved their autonomy and as a personal tax on large abandoned estates, often paid by peasants in produce. In post-conquest Egypt, most communities were taxed using a system which combined a land tax with a poll tax of 2 dinars per head. Collection of both was delegated to the community on the condition that the burden be divided among its members in the most equitable manner. In most of Iran and Central Asia local rulers paid a fixed tribute and maintained their autonomy in tax collection, using the Sasanian dual tax system in regions like Khorasan.