"Caesar" is the title officially used by the Sasanid Persians to refer to the Roman and Byzantine emperors. In the Middle East, the Persians and the Arabs continued to refer to the Roman and Byzantine emperors as "Caesar" (in "Qaysar-i Rum", "Caesar of the Romans", from Middle Persian "kēsar"). Thus, following the conquest of Constantinople in 1453, the victorious Ottoman sultan Mehmed II became the first of the rulers of the Ottoman Empire to assume the title (in "Kayser-i Rûm").