The Commentary, or Chain
Every folio has a literary component that has accompanied it on its journey. The Commentary is the tradition by which the Keepers interpret the images of the Codex, or interpret the Commentaries inherited from other Keepers (hence the "Chain") Because the Commentaries have been kept separate from the Folios, the Synapse is protected like a cipher that requires two keys. Without the folios (and the map each one contains) one cannot navigate the Labyrinth. Conversely, without the Commentaries, one might navigate the Labyrinth, but remain oblivious to the meaning of each room.