The development processes of open source software are different from traditional closed source development processes. Still, open source software is frequently of high quality. This raises the question of how and why open source software creates high quality and whether it can maintain this quality for ever larger project sizes. In this paper, we look at one particular quality indicator, the density of comments in open source software code. We find that successful open source projects follow a consistent practice of documenting their source code, and we find that the comment density is independent of team and project size. Categories and Subject Descriptors D.2.8 [Metrics]: Software Science General Terms: Measurement, Documentation, Languages