Discovering the Secret of the Mirror

In the stables, Adso's comment about Salvatore's Latin ('tertius equi') gives William the key to entering the finis Africae. He realizes that 'primum et septimum de quatuor' refers to the first and seventh letters of the word 'quatuor' in the verse 'Super thronos viginti quatuor' above the mirror. They rush to the library and press the letters 'q' and 'r', causing the mirror to swing open, revealing the entrance to the secret room.