Iranian covers have been invented based on the necessities and requirements of their time and have evolved gradually during centuries which has resulted in the creation of different types of them. In general, the covers are divided into two types of flat and curved ones. As a common curved cover in Iran, domes have had great influences not only on the geometric forms of buildings but also on the city perspective. Domes have been used in Iranian architecture more than other elements for their capability of covering a vast area. For the pressure existing among the dome components, any measure including renovation and strengthening must be taken based on the accurate and correct acquaintance with their overall nature and function; therefore, investigating domes and getting familiar with them not only reveals parts of Iranian architecture history but also assists us in proper renovation of these works. Previous researchers have studied and categorized Iranian domes but they have never been categorized extensively according to their shapes, main components and shell numbers. Considering the importance and use of domes in Iranian architecture, this research has been conducted at the aim of reviewing various types of domes and then presenting an extensive categorization based on their geometric forms, components and number of shells. Hence, this is the main question that how many types of Iranian domes there are based on the considered criteria. This research is a practical one and a compound research methodology (historical, descriptive and analytic) has been used to conduct it. The data has been derived from library and field studies (field observations of previous researchers have been used to obtain a comprehensive result.). What distinguishes this research from the previous ones is its variables, the research methodology and samples comprehensiveness. According to the results obtained based on domes shell number, there are 3 main types of dome. Each type in turn includes various sub-categories considering the space between shells, shell form (spherical, conical, combinatorial) and components (beam vault, opium poppy form, fasteners). Single-shell dome has 3 divisions: simple spherical, step-like, beamed domes, conical and domes with cupola. Double-shell domes have 3 main categories which include some sub-categories as well. Integrated spherical dome is the first type of double-shell dome which is the only separated double-shell dome. The second sub-category of double-shell dome is integrated hollow spherical dome which in turn includes simple, box-like, constraint, bladed and beamed forms. The 3th type of double-shell dome is the entirely disunited one which has 3 sub-categories: conical, spherical, combinatorial based on the outer shell form. Spherical dome also has some sub-categories: simple, onion, gothic, beamed, scalloped and zigzag. Double-shell conical dome includes 7 sub-categories: coneshaped, multiply, zigzag, scalloped, step-like, mound dome and combinatorial. For combinatorial dome two types of simple and beamed have been recognized. The last type of dome is 3-shell dome which has two sub-categories of spherical (simple and scalloped) and conical (simple, scalloped and multiply) considering their exterior shells. Totally, 27 types of dome have been recognized and categorized so far. Such form variety is the indicator of not only Iranian creativity but also acquaintance of Iranian master-workmen with the construction techniques and static issues.

Seyedun or Seyedun Ali Naghia tomb is located in Nebadan district of Abarghu. This clay monument is a tall dome chamber on the exterior cover of whose dome there are bricks as well as blue and azure tiles. The interior dado is also covered by tiles and the underneath surfaces are decorated with clay Muqarnas, inscriptions and slight painting ornaments. A vast part of the dome cover crumpled probably in 1330sSH (1950s AD) and was renovated some decades later. This building is a mosque attributed to 9th century (AH) called masjid-hosseiniyeh or hosseiniyeh of Nebadan. A small part of it which is part of the attached installations has just remained. It has had an altar whose direction confirmed to the skew Qibla of the congregational mosque and masjidhosseiniyeh of Abarghu. No dated inscription or including the founder name one can be found there. Seyedun Ali Naghia has been briefly inspected in the previous references and is attributed to 8th century (AH) based on Donald Wilber and Godard’s dating while it has been attributed to 6th- 8th centuries (AH) in the national registration file. In this research we try to date the tomb through investigating related documents, its structure and also the comparative inspection conducted during the preparation period of the renovation plan. In addition, we look for the answer to the question whether it was constructed in one single era and when it was built. Its initial function is another issue to be investigated. The dado tiles order, the way of drawing and embellishing Kufic handwriting of the inscriptions and the way of creating squinches in this monument to a great extent resemble 8th century monuments of Yazd and Isfahan. Moreover, the content of the verses written on the inscriptions which alludes to the burial use of them, in addition to above-mentioned traits, arouse the strong possibility of this monument being built as a tomb and for burial purposes. Both the structure and the ornaments are very similar to the damaged tomb of Hasan Ebn-Keikhosro (with an inscription dated 707-718 AH). Since the construction date of that tomb is known, it is likely that Seyedun Ali Naghia was also built in the first quarter of 8th century; the era during which king Mobarez Al-Din could overcome Yazd and Meibod that led to the reign of Mozaffarian dynasty to start. In addition, based on its similarities to 8th century monuments like Seyedun Golsorkhi and Hasan Ebn-Keikhosro domes and through investigating Jame’ Al-kheirat endowment in which Nebadan desert of Abarghu has been mentioned, we conceive that the mentioned buildings were all made in one single era with the fund of a famous figure who might belong to Seyed Rokn Al-Din family. Through investigating the structure of Seyedun Ali Naghia no trace of two different eras was found which leads us to the strong possibility that all of them were constructed in one single era. The content of the inscriptions read in this research for the first time, are Quran-related. We can foresee that more evidences particularly of the original forms of dome covers ornaments would be obtained through future renovations which might in turn reveal other aspects of the monument history.