Whereas the main body of the cathedral is Romanesque in style the easterly part, built at a later stage, is gothic in style.
The people of Tournai wanted to have their own bishop with his own diocese
and while various popes opposed the wishes of the people of Tournai
the town repeatedly enlarged their church so as to convince the pope to install a diocese around Tournai.
This long construction history meant that architectural styles evolved:
Romanesque towards the west and Gothic towards the east, with an in-between style for the transepts.