A mediaeval Irish monk was able to switch effortlessly between Irish and Latin when writing a book. This phenomenon of code switching is common to all eras; we switch effortlessly between Dutch and English nowadays too. Nike Stam researched this phenomenon by studying mediaeval calendar texts. She concluded that the way in which these monks switched languages in their writings was akin to the way we switch languages in our conversations. She defended her PhD thesis at Utrecht University on Wednesday, 7 June. Her research was made possible by funding from the NWO Free Competition programme.