Certain igneous rocks frequently shew division planes which meet each other in such a way as to form a series of polygonal prisms.
"Geology"
James Geikie
The base consists of a series of mouldings and frequently stands upon a double or single plinth, which in the earlier examples is square, but in later examples assumes the form of the base, and is either circular or polygonal.
"Our Homeland Churches and How to Study Them"
Sidney Heath
Some rectangular virginals were made in Italy; one Flemish polygonal virginal, made by the elder Hans Ruckers in 1591, survives.
"Italian Harpsichord-Building in the 16th and 17th Centuries"
John D. Shortridge