The Ulu Camii or Great Mosque of Bursa is a venerable and quite pleasant building, dating from 1396. The Lonely Planet guide compares it to the Erzerum Ulu Camii, but I disagree, that one is much darker and though nice does not give me the delight I always feel in the Bursa one.

It was the first large congregational mosque of the Ottomans and measures 56 x 68 m over all.
The twenty domes rest on pendentives (not Turkish triangles) and twelve heavy piers.
Supposedly the twenty domes were built instead of the twenty (separate) mosques which Sultan Bayezid I had promised for winning the Battle of Nicopolis in 1396.

A ‘pendentive’ is a constructive device permitting the placing of a circular dome over a square room (or an elliptical dome over a rectangular room). The pendentives, which are triangular segments of a sphere, taper to points at the bottom and spread at the top to establish the continuous circular or elliptical base needed for the dome. In masonry the pendentives thus receive the weight of the dome, concentrating it at the four corners where it can be received by the piers beneath. The pendentive was introduced by the Romans (around 200 AD) and fully used by the Byzantines.

The ‘Turkish triangle’ was a new pendentive: a transformation of the curved space of the traditional pendentive into a fanlike set of long and narrow triangles built at an angle from each other. It was introduced in Anatolia by the Seljuk.

Correspondent: J.M.Criel, Antwerpen.
Source: ‘Islamic Architecture: Ottoman Turkey’ (Godfrey Goodwin) – London 1977 - Wikipedia
& Encyclopaedia Brittanica .