Arabic letters on papyrus challenge the modern reader. There are few to no diacritical dots to distinguish homographs, no systematic spacing between single words, and in the majority of cases a low degree of graphical structuring. However, contemporary readers usually read and understood these documents easily – probably because the recipient of a letter knew what to expect. The letters are formulaic, and their information packaging follows an algorithm typical for their time and content. Here formulaic letter writing means not only the reuse of the same formulae or topoi but expressing thoughts in a predictable linguistic way and order, both as a matter of readability and as one of adequacy and politeness. The main concern of this work is to discover these unwritten rules and norms behind Arabic letter writing on papyrus.