Investigation of paper discolouration in a large number of books showed a clear connection between individual stains and other more extended forms of paper browning. On this basis a redefinition of the phenomenon of foxing is proposed. Further observations indicated that foxing appears to develop as a spatial process in any stack of paper in the course of book production, leading to matching stain patterns on sequential and non-sequential book pages. As the current explanations for the origin of foxing cannot satisfactorily explain our findings, another approach is suggested. We presume that the discolouration of paper primarily develops at sites of temporary moisture accumulation, caused by local condensation processes in the book, and is probably closely connected with the known browning reaction which takes place in humidified fibrous materials at the dry-wet interface.