The dome has been vital to the development of Persia’s great architectural achievements, as well as to world future architecture.
In its various forms the vault has been without doubt the most important element in Persian building.
Its widespread adoption was necessitated by the lack of sufficient wood and timber to continue the Achaemenid habit of post and lintel construction.
Vault construction was already in use from very early times ( Choga Zambil entrance vaults date from 1200 B.C.).
Among the three main types of vault, all derived from a single elemental component (the arch), the hemispherical vault, commonly called the dome, is the most complex one and it is capable of innumerable variations.
In the dome the main problem is how to manage the transition from square plan below to circle above. For centuries this problem baffled the very competent Roman engineers, who did not provide attractive and assured solutions, while Persian engineers and masons realized a third section, a zone of transition between the square chamber below and the round dome above, by building an arch across each corner and reducing the square to an octagon with a further ring of small arched panels to reduce the eight sides to sixteen , which is close to being a circle.
The dome took then many forms in Persia: round, ovoid, parabolic, bulbous on cylindrical base, onion shaped, low saucer-like, high on crown circular and octagonal structures.