duodecagonal

[…]the Luſitanian Cicero (as his countrymen call him) proceeds to tell his audience, that “if the Supreme Being was to ſhow himſelf under any geometrical figure, it would certainly be the circular in preference to the triangular, the ſquare, the pentagonal, the duodecagonal, or any other known to the geometricians.

1770, Joseph Baretti, A Journey from London to Genoa, Through England, Portugal, Spain, and France, volume I, Dublin: […] T. Ewing, […], letter XXIX, page 132

[…]the town-house is splendid, it is built on a public square, adorned with a tower and clock, similar to that of Lyons, a high column with a brazen statue of the Blessed Virgin, entirely gilt, a large water reservoir of a duodecagonal figure, and with a statue on a pedestal in the centre;

1806, Denis Louis Cottineau, “Germany”, in Geographical Compilation for the Use of Schools; Being an Accurate Description of All the Empires, Kingdoms, Republics and States, in the Known World: […], volume I, Baltimore, Md.: […] John West Butler, section third (Europe), article tenth (Bohemia), paragraph second (Kingdom of Bohemia), page 504

Every monastery and cathedral had a chapter house. They were nearly always rectangular and were the most important building after the church itself. Thirty centrally planned chapter houses were constructed between 1100 and 1500: although about two thirds were octagonal, there are circular, decagonal and even duodecagonal examples.

1999 February 3, “Notes & Queries”, in The Guardian, London, page 17, column 3