Caudex is a Latin word meaning tree trunk but, towards the end of the 1st century A.D. it was used as a term to indicate a new format used for documents kept not in the usual papyrus rolls but in parchment codices (term deriving from the Latin caudex) written in two or more columns in recto and verso and then folded and sewn together.

Caudex, together with the terms liber, libellus and codex, was used by the Romans before the invention of the codex, when wood boards were used to write notes, records or speeches. Between the 1st and 2nd century A.D., after the birth of the codex, these terms were applied to the new format in contrast with the volumen that referred to the classical roll.