At-Tirmidhi's collection contains just under four thousand traditions arranged by legal and thematic chapters, but its lasting contribution is what he wrote after them. Following a report he will typically remark that it is sound, or good, or good and sound, or strange, meaning transmitted by a single narrow channel, sometimes combining the terms in formulas whose exact force scholars still analyse.
The middle grade, hasan, was his signature. Earlier critics had worked with a sharp division between sound and weak; at-Tirmidhi gave systematic standing to the report whose transmitters fall a little short of the highest accuracy yet whose content is supported from elsewhere, good enough to establish practice though not the strongest proof. He explained his terminology himself in the Ilal, the discussion of hidden defects appended at the end of the collection, which became a foundational text for the science of hadith criticism. Alongside the grades he records the fiqh: after a tradition on prayer or sales he notes that some of the people of knowledge acted on this, that Sufyan al-Thawri held one position and the people of Medina another, and what Ahmad and Ishaq preferred. A reader therefore learns not only what the Prophet is reported to have said, but how strong the report is and what the schools made of it, three layers of information in one book.