All mathematical writing has a reader. Sometimes the reader is another person; sometimes it is the author, days or years later. The two call for very different kinds of writing, and two famous notebooks show what happens when a text written for oneself falls into other people’s hands. Gauss’s diary On 30 March 1796, an 18-year-old student in Göttingen opened a small notebook and noted, in Latin, that he had discovered how to divide the circle into 17 equal parts with straightedge and compass, a problem open since the Greeks. It was Carl Friedrich Gauss, and it was this result that made him choose mathematics over philology. Over the next eighteen years he wrote 146 short entries in the notebook, almost always a single sentence, sometimes a formula.