Northern Iran faces the Caspian Sea--the world's largest lake--in a curving sea from the Azerbaijani border to Turkmenistan. There is a populous lowland plain, generally quite narrow and contained by the Elburz Mountains except in the northeast. These mountains reach over five and a half thousand meters at Mount Damavand, about 60 kilometers northeast of the national capital. Beyond the mountains this area includes--in the south and southeast--part of Iran's vast central plateau. At the south central edge is a large marsh, and to its east is the great Salt Desert(1)--extending southward beyond the area.
About 20 million people live there speaking a variety of languages. The two leading languages together constitute a majority but it is difficult to say how may speakers there are of each. Presumably there are more Persian speakers than Azeri(2). About one in six speak Gilaki and about the same number, Mazandarani. Less than one in ten speak Turkmen. (Persian, Gilaki and Mazandarani are all Western Iranian languages which certainly account for a majority of local speakers.) Most everyone is a Moslem, and all of these who are not Turkmen are Shi'ite; the Turkmen are Sunni.
The capital, Tehran, is Iran's only city with more than ten million metropolitan inhabitants, including a million plus in its satellite city, Karaj.
Far south-southwest of Tehran is Qom, the chief city of Markazi--though not its capital. Recently its nature has become industrial, but historically it was religious: a pilgramage site for Shi'ites featuring the golden-domed shrine of Fatima.
People have been here for as long as they have anywhere else in mainland sub-arctic Asia--perhaps nearly 100,000 years.
By the end of the first millenium B.C.E. Old Persian was spoken in the area, having arrived from what is now Kurdistan(3). However Aramaic was the primary language used by political and economic elites, although Greek also shared this role for a time. Later Parthian took over this role until it too was superceded, giving way to Pehlevi(4), which also replaced its ancestor, Old Persian, as the everyday language. When Arabs conquered Arabic became the elite language and also--by the tenth century--transformed Pehlvei into Modern Persian.
The ancestors of the Azeri and Turkmen arrived from the north between the sixth and the 11th centuries, speaking the language that fathered Turkmen, Azeri and Turkish.
Gilaki and Mazandarani have been spoken along the Caspian Sea for at least hundreds of years. Like Persian and Parthian they are Iranian languages.
The series of religions here was similar to Iran as a whole: I. Practices dervied from Ino-European roots; II. Zoroastrianism, introduced from the north, and eventually becoming a state religion; III. Christianity and syncretic blends of it with Zoroastrianism developed alongside anadulterated Zoroastianism; IV. Islam took the area by conquest in the middle of the first millenium and largely exterminated the older religions. I'm not sure how the area became Shi'ite: whether in the original schism or later. While the Azeri, probably began as Sunni like their relatives, the Turkmen, they converted to Shi'ism early on.
northwest
northeast
west of the province's north
southeast of Markazi Province and south of Qom and Semnan Provinces
west and southwest of Markazi Province
west of Ardabil; southeast, northwest and west of Zanjan; southwest of Tehran; and west of Qom
(1) Dasht-e Kavir in Perisan.
(2) Ethnologue.com says Iran has 23.5 million "Southern Azerbaijani" speakers but Dalby's Dictionary of Languages gives a total for all countries for all forms of Azerbaijani of 14 million, and allows an Iranian share of at most 8 million. Ethnologue has no map but Dalby's map confines them to the northwest provinces including part of this area. Allowing for Kurds about six milion are accounted for in the two Azerbaijani provinces, Kordestan (Kurdistan), Zanjan and Qazvin. That leaves two to 17 or 18 million. Hamadan can account for less than two million, leaving from less than one million to 15 to 16 million remaining. I'm assuming Dalby is somewhat low and Ethnologue very high. If there are two million speakers here, then Persians are less than a majority in the area. If Azeris here are less than about six million, then Persians outnumber them.
(3) Kordestan in transliteration from Persian.
(4) Also called Middle Persian or Pazand.