The dome on squinches first appeared in Persia in the Sasanian period in the palace at Firuzabad (qv) in Fars and at nearby Qal´a-ye Dokhtar, both erected by Ardeshir (224-40) in the early third century.

Although the dome chambers at these sites are impressive in size, their conical squinches (arches across the corners of a cube, forming a zone of transition) are crude in design and execution, CAIS reported.
The rubble masonry was so haphazardly applied that it is difficult to distinguish the outer edges of the squinches from the corbeled walls between them.
The extreme thickness of the walls in proportion to the height of the dome is another indication that these chambers stood at the beginning of the series.
There is no evidence of precursors. Roman domes on circular bases or smaller domes on pendentives obviously sprang from a different tradition.
Evidence for the simpler pitched-brick dome exists from as early as the ۳rd millenium BCE in Mesopotamia, but the absence of any known intermediaries between them and the Firuzabad domes, even from the Parthian period, attests to the originality of the Sasanian examples.
The most numerous surviving Sasanian domes are those over chahartaqs, frequently the central chambers of fire temples.
It is in the large dome of the structure variously identified as a palace and a fire temple at Sarvestan in Fars province that two constant features of later dome design first become apparent: the use of lighter materials, in this instance brick, for the dome itself and decorative emphasis on the zone of transition, which at Sarvestan is carefully set off by dogtooth moldings above and below and lightened by four windows between the squinches.
The squinches are also more clearly articulated, each consisting of intersecting segments of two tunnel vaults. These features are, however, consistent with an early Islamic rather than a late Sasanian date for Sarvestan, a possibility also mooted for several chahartaqs.
The survival of the dome on squinches in the Islamic period may have been encouraged by the transformation of several chahartaqs into mosques, documented in a few instances.
The building of isolated dome chambers to serve as mosques (“kiosk mosques”) may have followed, though the evidence comes not from the usually cited congregational mosques of major cities but from a few, mostly Saljuq examples in small villages in Khorasan.

For Shia Martrys
It was the appearance of the domed mausoleum that was to prove most important for the development of the dome in early Islamic Persia.
Despite initial Islamic hostility to tomb structures, by the ۱۰th century several had probably been built by the Abbasid caliphs for themselves.
Domed tombs had also been erected over the graves of many Shi´ite martyrs, and the respect paid to them by pilgrims may well have hastened the spread of the form.
Two early mausoleums in Transoxania stand out: that of the Samanids in Bukhara and the ´Arab Ata at Tim, between Bukhara and Samarqand.
In the zone of transition in the former the squinches are of the same width as the arches between them, resulting in a regular octagonal plan at this level; this scheme was followed in virtually all later examples.
At Tim the zone of transition is further unified by trilobed squinches composed of large moqarnas (oversailing courses of niche sections) separated by trilobed arches.
Tomb towers represent another tradition in Persian mausoleums; many survive from the beginning of the ۱۱th century onward.
They are circular or polygonal in plan, thus reducing the importance of the zone of transition, and they frequently have conical roofs masking interior domes. Although the domes of the earliest tomb towers are plain, the shafts often display the inventive decorative brickwork of the period, as at the Pir-e ´Alamdar and Chehel Dokhtaran at Damghan. Two other notable examples are at Kharraqan on the Qazvin-Hamedan road ۳۳ km west of Ab-e Garm; the earlier had the first masonry double dome known from Persia.

Saljuq Period
The introduction by the Saljuqs of a maqsura (enclosure) in front of the mihrab of the hypostyle mosque helped to transform the skylines, characteristically punctuated with domes, of Persian towns.
Although in the early Islamic period dome chambers may have been used for small neighborhood mosques, it was only after the vizier Nezam-al-Molk introduced the domed maqsura in the congregational mosque at Isfahan that dome chambers on the qebla become the norm in Persian congregational mosques.
The numerous Saljuq dome chambers of northwestern Persia usually have much simpler zones of transition than those at Isfahan but, perhaps in compensation, are abundantly decorated with carved stucco and are also sometimes articulated on the exterior.

Il-khanid Era
The dome chamber of the congregational mosque in Varamin provides an example of changes in Persian domes in the Il-khanid period.
Its taller proportions result primarily from the increased height of the zone of transition, with the addition of a ۱۶-sided zone above the main zone of moqarnas squinches.
Extra light enters through eight windows in this upper tier, although, as the Persian climate necessitated avoidance of sunlight during most of the year, architects of Persian dome chambers never aspired to the walls of light that characterized Ottoman examples.
The major Il-khanid domes were those of the mausoleums of Guazan Khan at Tabriz and Oljayto at Soltanie, each at the center of a larger complex of buildings.

Design Revolutionized
After the turn of the ۱۵th century the Timurids built very few freestanding mausoleums, attaching them instead to madrese (religious schools), often in pairs.
The dome chambers erected within these madrasas revolutionized the design of interiors, as in the madrese of Gowhar Sad at Herat and the madrese at Khargerd.
This change probably originated in ۱۴th century experiments with small lantern domes set at right angles to the main transverse vaults, from which arose the concept of using intersecting arches to support a dome with a diameter smaller than the width of the square below.
The chamber was also modified, with a deep recess added to each side to produce a cruciform plan. The result was a much more fluid space than had been possible with the rigid tripartite division of Lower Square, zone of transition, and dome. In particular, the intersecting arches provide a visual link between the dome and the dado level, an illusion, as the thrusts are taken up by concealed masonry.
In the madrese of Gowhar Sad the dome is actually a triple shell, the first of its kind; the intermediate dome presumably was added for reinforcement.
The tilework of the Qara Qoyunlu Mozaffariye mosque (also known as the Blue Mosque) at Tabriz is outstanding. Above a marble dado the whole of the interior of the dome chamber on the qebla was faced with dark-blue hexagonal tiles with stenciled gilding, creating a richness that was unparalleled until construction of the mosque of Shaikh Lotf-Allah in Isfahan, the quintessential Persian dome chamber.
In the latter the blending of the square, zone of transition, and dome was achieved by unifying the first two, rather than the upper two, as in the Timurid examples with intersecting arches.
The form of the squinches is plain, recalling those of Sarvestan and the Saljuq domes of northwestern Persia, but the way in which the framing arches and the enclosed tilework patterns continue in an uninterrupted sweep down to dado level was an innovation.
The four identical arches between the squinches are edged by a bold twisted turquoise cable, lending the interior a new unity and simplicity.
The exterior of the dome displays another innovation, the use of multiple levels of arabesque, interwoven with the brick ground with such finesse that it has often been mistakenly presumed that the ground, too, was glazed.
The domes of the ۱۷th-century Masjed-e Shah and Madar-e Shah madrese in Isfahan show how effective a similar arabesque pattern can be against a light-blue tiled ground.
In Transoxania and neighboring regions the Uzbeks carried on the Timurid tradition of dome building with little change, though dome chambers were sometimes surrounded with axial eyvans and corner rooms, as in the khanaqah (Sufi monastery) of Qasem Sheikh in Kerman.
Where these corner rooms are part of an octagonal plan and on two stories, as in the shrine of Khuaja Parsa at Balkh, the form was a prototype for the major Indian mausoleums of Persian inspiration, that of Homayun in Delhi and the Taj Mahal.
The domed timchas (markets) that survive at major intersections in Shaybanid Bukhara are among the earliest survivors of the type.

Vernacular Architecture
Throughout Persian history the dome also played an important role in vernacular architecture. On many parts of the Persian plateau where wood is scarce whole villages with domed roofs are to be observed.
From Sasanian to Qajar times caravansaries were frequently constructed with the domed bay as the module.
Domed cisterns and icehouses (yakchals), still common sights in the Persian countryside, are other reminders of the variety, pervasiveness, and permanence of the dome in Persian history.
 By: Bernard O’Kane