(1) The counterpart to elevation, with all its aforementioned categories, is descent. Thus every category of elevation is met by (2) a corresponding category of descent, contrary to those who maintain that elevation may occur without being linked to any descent (3).
[Peer Transmission]
If the narrator and the person from whom he transmits share some attribute connected to hadith transmission—such as (4) age or (5) having met the same teachers, which (6) is the act of receiving from the shaykhs—then the report falls under what is called the narration of (7) peers, because in that situation he is narrating (8) from someone who is his equal.
[al-Mudabbaj]
When each of the two peers narrates from the other, the report is termed al-mudabbaj. This is more specific than the previous category: every mudabbaj is peer transmission, but not every peer transmission is mudabbaj. Al-Daraqutni authored a work on the subject (9), and Abū Shaykh did likewise.