They mention the possibility that this was because there were two emperors at that time (in Constantinople and Rome), but also mention that "plurality is a very old and ubiquitous metaphor for power". This usage was extended to other powerful figures, such as Pope Gregory I (590–604). However, Brown and Gilman note that it was only between the 12th and 14th centuries that the norms for the use of T- and V-forms crystallized.