I swore by the Seven that were lengthened, ... and by Hundreds that were made hundreds thereafter, (1) And by Mathānī that were doubled and repeated, ... and by the Ṭawāsīn that were made three, (2) And by the Ḥawāmīm that were made seven, ... and by the Mufaṣṣal that were divided! (3)

Abū Jaʿfar, may Allah have mercy on him, said: These verses demonstrate the correctness of the interpretation that we have given to these names.

As for "the Mufaṣṣal": they were named mufaṣṣal because of the frequency of the divisions between their surahs with "In the name of Allah, the All-Merciful, the Most Merciful".

Abū Jaʿfar said: Then every surah of the surahs of the Qurʾān is called a "sūrah", and its plural is "suwar", on the pattern of "khuṭba / khuṭab" and "ghurfa / ghuraf".

A sūrah, without hamza, is a rank or high station. From that is the wall (sūr) of a city, so named because of the wall that encompasses it due to its elevation above what it contains. However, the plural of sūrah (wall segment) from a city wall has not been heard as suwar, as has been heard for the plural of a sūrah of the Qurʾān as suwar. Al-ʿAjjāj said regarding the plural of sūrah in building:

Many an owner of an enclosed tent ... I traveled to at the heights of the walls! (4)

Thus, the grammatical pattern of its plural turned out according to the pattern of the plural of burra (grain of wheat) and busra (unripe date), because those are pluralized as burr and busr. Likewise, sūr has not been heard as a plural for a sūrah of the Qurʾān, although if it were pluralized thus, it would not be an error by analogy if all of the Qurʾān were intended by it. They only abstained—in our view—from pluralizing it in that way because every collective noun that shares the form of a singular masculine noun, such as burr (wheat), shaʿīr (barley), qaṣab (reeds), and the like, then