How did the decretal Per venerabilem become part of canon law?

The decretal Per venerabilem, written by Pope Innocent III in 1202, became part of canon law because it was incorporated into the official collections of papal decretals that followed Gratian’s Decretum.12

While decretals were originally issued as specific replies to individual questions of church discipline and did not inherently possess the force of general law, they gained immense influence through their application to analogous situations. Over time, these decisions were gathered into various collections, such as the Quinque compilationes antiquae. Eventually, these compilations served as the primary source material for the Liber extra (or Liber decretalium Gregorii IX), which was compiled by St. Raymond of Peñafort and promulgated by Pope Gregory IX in 1234 as the exclusive codex for all canon law following the Decretum Gratiani. Through this systematic process of compilation and official promulgation, Per venerabilem—which established the pope's right to evaluate imperial candidates—was codified into the broader body of ecclesiastical law.312