Tarom County () is in Zanjan province, Iran. The capital of the county is the city of Ab Bar. At the 2006 census, the county's population was 42,939 in 10,734 households. The following census in 2011 counted 46,616 people in 13,221 households. At the 2016 census, the county's population was 46,641 in 14,438 households. Administrative divisions The population history of Tarom County's administrative divisions over three consecutive censuses is shown in the following table. The latest census shows two districts, five rural districts, and two cities. History The region of Tarom is historically divided into two parts: Upper Tarom and Lower Tarom. Medieval Arabic geographers usually wrote the name as aṭ-Ṭārumayn, or "the two Taroms", reflecting this division. The mountainous Upper Tarom was historically counted as part of Daylam. The name "Tarom" was applied to a right-bank tributary of the Sefid Rud, and the region of Tarom comprised the river and its own tributaries. An important location in Tarom was the castle and town of Semiran, which lay in Lower Tarom on the main highway leading to Sarab. The castle crowned a rocky mount above the lower town and had triple walls. Ibn Muhalhal visited here 943 and wrote that it was one of the main strongholds of the Daylamite kings and had about 2,850 houses. The Buyid amir Fakhr al-Dawla captured Semiran in 989 from the Vahsudan dynasty. Al-Muqaddasi wrote about the same time that the fortress of Semiran had "lions of gold, and the sun and the moon" on its walls; the town's houses were built from mud brick. He counted Semiran as part of the Salārvand district. Nasir-i Khusraw visited Semiran in 1046 during his pilgrimage to Mecca; he described it as the capital of Tarom. Its fortress, he wrote, was garrisoned by a thousand men and had an underground conduit to supply water. By the time Yaqut al-Hamawi visited Semiran in the early 1200s, the castle had been slighted by the Nizaris, although the ruins were still impressive enough that Yaqut called it "a mother of castles". Another fortress that Yaqut mentioned was Qilāt, located in the mountains on the Daylam frontier. It was located atop a mountain and had belonged to the Nizaris of Alamut. Below it was a town with "excellent" markets; there was also a masonry bridge with many arches crossing the stream. In the Nuzhat al-Qulub The 14th-century author Hamdallah Mustawfi gave