The Mosque of Nur al-Din in Mosul is one of the city's three great mosques and is also known as al-Jami' al-Nuri al-Kabir (The Great Nuri Mosque). Located in the heart of the old city, construction on the mosque began in ca. 1170/566 AH under Nur al-Din Mahmud ibn Zangi (r. 1146-1174/541-569 AH) and included both a prayer hall and a madrasa. The mosque is perhaps most famous for its gently leaning brick minaret, known as "al-Hadba," which is situated on the north side of the complex. The mosque was irrevocably altered during 1942 as part of a renovation project orchestrated by the Iraqi Department of Antiquities. These restoration works consisted of destroying the old mosque and rebuilding it with old and new material according to a new plan. All that remains from the original complex are the minaret, two mihrabs, an inscribed marble slab, and some stucco decoration. 

The present-day mosque consists of a rectangular prayer hall with a dome covering the central bay before the mihrab. Adjoining this prayer hall to the north is a large open courtyard with an ablutions fountain. The western part of the courtyard is paved while the eastern part contains greenery. 

The original form and decoration of the mosque and its sequence of construction is a matter of some debate. Yasser Tabbaa has argued that the mosque originally took the form of a rectangular, shallow prayer hall, seven bays wide and one and a half deep, with a dome above the central bay in front of the mihrab, not unlike the structure that still existed in the early twentieth century before the 1944 demolition.1 The prayer hall would have opened on its north side onto a large open space, perhaps surrounded by a wall. The size of this space was most likely larger than the present courtyard or that of the early twentieth century mosque.2 Ernst Herzfeld, who conducted the only scholarly study of the mosque prior to its demolition, argued for a deeper prayer hall with bays that alternated between smaller and larger rectangles, some vaulted and some domed. His argument for the depth of the prayer hall was tentative, however, and largely based on the remains of columns in the courtyard north of the structure that existed in 1910.3 

As far as chronology, it is clear that the mosque underwent several phases of building before its 1940s reconstruction. The most obvious piece of evidence for this are the existence of two types of columns in the prayer hall. Yasser Tabbaa summarizes the argument in his 2002 article on the mosque.5 Herzfeld believed that the two column types represented an initial phase that pre-dated Nur al-Din followed by a renovation by Nur al-Din. This argument was based on the fact that the mosque's stone mihrab bore the date 1148 and that the decoration on one group of columns was comparable.6 The validity of this statement was contested as early as 1949 by Daywahji, who pointed to the fact that the mosque's mihrab, which Herzfeld assumed was original to the mosque, was most probably transferred from the Umayyad Mosque of Mosul (the city's original Great Mosque), to Nur al-Din's Mosque at the time of construction, eliminating the necessity for backdating the column group and construction of the mosque to 1148.7 Tabbaa suggests that the installation of the mihrab and its corresponding columns dates to the period of Nur al-Din while the later columns were added as a form of reinforcement at a later date, harvested from local Christian sites that may have fell into disuse.8

It was reported that the mosque and minaret were destroyed on June 21, 2017 in an explosion.9

Notes:

Sources:

Daywahji, Sa'id. Jawāmi‘ al-Mawṣil fī mukhtalif al-‘uṣūr. Baghdad: Maṭba‘at Shafīq, 1382 [=1963].

Sarre Friedrich and Ernst Herzfeld. Archäologische Reise im Euphrat- und Tigris-Gebiet, 2:216-234. 4 vols. Berlin: D. Reimer, 1911-1920.