Al-Tabari's full title announces the project: the comprehensive exposition of the interpretation of the verses of the Quran. For each verse the commentary proceeds in a fixed rhythm. The verse is cited, the possible readings of its Arabic are set out, and then the transmitted interpretations arrive, each introduced by a chain of authorities running back to a companion of the Prophet such as Ibn Abbas, or to an early interpreter such as Mujahid, Qatada or al-Suddi.
Where the traditions disagree, al-Tabari lists the positions as numbered opinions, argues from grammar, from the usage of the Arabs and from parallel verses, and then declares which reading he holds most correct, in a formula that recurs thousands of times. This willingness both to preserve every inherited opinion and to adjudicate among them is what gave the work its double character, at once an archive and a judgment.
Later commentators mined it in both capacities: those who wanted the early tradition found it stored here with its documentation intact, and those who wanted a master's verdict found that too. Because so many of the early written sources al-Tabari drew on subsequently vanished, his commentary became the principal surviving witness to the first age of Quranic exegesis, and modern scholarship still reconstructs lost works from his quotations.