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Encyclopedic overview
Onion domes of Saint Basil's Cathedral
An onion dome, also known as onion cupola, is a dome whose shape resembles an onion. Such domes are often larger in diameter than the tholobate (drum) upon which they sit, and their height usually exceeds their width. They taper smoothly upwards to a point.
Excerpted from Wikipedia’s “onion dome” article, available under the CC BY-SA 4.0 licence.