This paper gives an example of how enriched diachronic treebank data can shed new light on an old and conflicted topic, even when that topic is primarily morphological and semantic in nature rather than syntactic. The topic is the rise of the Russian po delimitatives, a change seen as crucial in most accounts of the history of Russian aspect, since it represents a major step in generalising the derivational aspect system. Earlier accounts concur that the po delimitatives spread fairly recentl...