Half the north facade (the part to the right of the main entrance), with its open loggias on the first floor.
The mosque is built out of sandstone and clad with marble panels, a majority of which was replaced in the mid-19th century, when the building underwent an extensive renovation led by the French architect Léon Parvillée, as Ahmet Vefik Paşa, the Vali (governor) of Bursa, was unable to find a qualified Turkish architect.

The presence of the outer mihrab (prayer niche) is related to the porch (gallery) that had been planned, but was never built, because Mehmet I died before the work was begun.
The porch was intended to be the ‘son cemaat yeri’ of the mosque (= ‘area for those who came late’), where worshippers who reached the mosque late or at the last moment, will gather and fulfil their prayer duty. The presence of secondary mihrabs makes those worshippers feel more connected to the congregation inside.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Vakıf Abideler ve eski Eserler’ - Vakıflar Genel Müdürlüğü III, Ankara 1983 - Wikipedia,