Conventionally part of the composite letter 13.17 (or “17–18”) in every modern editorial tradition — the Latin Library, Perseus, and Shackleton Bailey all print the material as a single letter under the lower number. The split numbering “13.18” survives in older citation conventions (Tyrrell–Purser and a handful of nineteenth-century commentators), and is preserved here as a citation anchor so that scholarly references to “Ad Att.~13.18” still resolve. The text itself appears in this edition under ../ad-atticum-13-17/Ad Atticum 13.17.