Although comma usage is one of the most clearly regulated areas of orthography, it still poses significant challenges for even experienced writers. On the other hand, many writers are very proficient in using commas without being able to specify the underlying rules. They use commas 'by instinct.' This study investigates which linguistic features are responsible for this type of comma usage and how they influence comma placement behavior. Apparently, writers rely on criteria that are quite different from those that orthographic didactics have so far considered dominant. In particular, semantic and intonational factors play a significantly larger role than, for example, syntactic ones that underlie the official comma rules.