The Gregorian melodies are made up of short melodic fragments that are joined together like a patchwork quilt in a technique known as centonization. The more one becomes acquainted with the Gregorian repertory, the more these familiar melodic fragments begin to reveal themselves. Sometimes a substantial portion of one chant (or even the entire melody) is borrowed by the Gregorian composer for use with completely different words. This leads to a technique known as adaptation, where an existing melody is modified to accommodate a new text.
Read more about Centonization.