The fifth fascicle of the supplement to CIL IX contains addenda and corrigenda on inscriptions that were edited by Theodor Mommsen in 1883 as well as those published by Marco Buonocore in 2018–2022 in Fasc. 1–4. It also documents around 150 recently found Latin inscriptions from the Regio Italiae quarta. Indices to the main volume and supplement round out the volume, which is enhanced with an overview map.