or the break may be hidden, discernible only to the master scholars who are thoroughly versed in the various transmission routes and in the subtle defects (1) found in isnāds.

As for the first type—the obvious one—it is detected by the absence of any meeting between the narrator and the teacher from whom he claims to report: either because they never lived in the same era, or because, although their lifetimes overlapped, they never actually met, and the narrator possesses from him neither an ijāzah (formal authorization) (2) nor a wajādah (3).

For that reason the science of historical chronology became indispensable (4), for it records with precision the birth- and death-dates of the transmitters, as well as the periods during which they sought knowledge and undertook their journeys.

Indeed, certain people (5) who claimed to transmit from particular shaykhs (6) have been exposed, for the historical record revealed the falsehood of their claims (7).

[The Mudallas]
The second category—the hidden one—is the mudallas (8) (with the lām pronounced with a fatḥah). It is so called because the narrator omits naming the person who actually related the hadith to him, thereby creating the impression that he heard it from someone who in fact never transmitted it to him (9).