The analysis of a historical mathematical textbook constitutes a particular challenge when the original had been composed before the invention of the printing press and an even greater one when the very original might even not have been fixed as a manuscript. The analysis of such cases affords a focus on the relation between orality and written text. A particular clue for this relation is given by the diagrams accompanying the text. Diagrams in mediaeval manuscripts have several particularities which have been eliminated in modern editions as inappropriate for mathematical arguments. However, they may have not been so strange in the context of the direct oral teaching in which the teacher drew the diagrams in front of the pupils. The text and the structure of the Elements also contain other particular features preserved also in modern editions of which the following ones are examined: (1) no use of proposition numbers, (2) label assignment in alphabetical order which may result in different assignments in similar propositions, (3) oscillation of the appellation of the objects expressed by two or more letters such as line AB and line BA, (4) long and sometimes incomprehensible protasis (general enunciation at the beginning of each proposition). All these particularities can be interpreted as traces of oral teaching and communication of mathematics in ancient Greece.