jats:titleAbstract</jats:title> jats:pMedieval letters from the Cairo Geniza can be broadly classified into private, official, or mercantile correspondence, and all use particular linguistic registers. Official correspondence, for example, shows abundant code switching into Hebrew and the employment of high-style versus lower-style prose. Mercantile letters actively avoid Hebrew and emulate supraconfessional Arabic writing standards. Private letters typically display more colloquial and less standardized forms than other genres and are more often written in crude handwriting. Among these private letters, we find one written by or for women that share common features of colloquiality and less standardization even when they are transcribed by male scribes. Linguistic registers are also influenced by the time and place in which they are written, and comparing Geniza letters from different areas and time periods exposes geographic and chronological characteristics. For example, North African letters tend to be linguistically more conservative, and Babylonian and Egyptian letters show differences in layout and style. Throughout the medieval period, orthographic, grammatical, lexical, and stylistic changes in the letters reflect social and economic evolution over time. The principal trend is a distinct move away from prescriptive Arabic linguistic norms from the late twelfth century on.</jats:p>