Abstract The paper presents the approach referred to as the ›No Blur Principle‹ (NBP), aka ›Vocabular Clarity‹. This approach can account for an otherwise unexpected case of diachronic productivity in Faroese verb conjugation, viz. the spreading of the suffix -i in the present tense 3rd sg., not associated with the largest class. Even if the NBP is meant as a synchronic restriction, the paper shows some consequences for diachrony – yielding the right ›predictions‹ (in a weak sense of the word). Given the NBP analysis of Old Norse, there is a good reason why the suffix -a, at first sight a much better candidate for productivity (as it is associated with a more type-frequent class than -i), could not spread – at least not without extensive ›reshuffling‹ of the affixes first; no such reshuffling was necessary for the spreading of -i, which was the ›class-default‹ affix anyway. The traditional account of the spreading of Faroese -i,which goes back to Rask (1811, p. 279), has it that this spreading is due to influence from the subjunctive. This is problematic, because one would not normally expect affixes to spread from the subjunctive to the indicative, and because a similar development in some few dialects of Norwegian cannot plausibly be accounted for along such lines. While the NBP account does not exclude Rask’s hypothesis, it can help us overcome these problems. The fact that the NBP helps us answer questions of productivity must count in its favour, and some other arguments in its favour are also marshalled. The Faroese development also shows that productivity for affixes and words need not behave in identical fashion; thus supporting a theoretical distinction. Thanks to audiences in Oslo (ICHL 21, August 2013), Zürich (Doktorierendenkolloquium Linguistik, October 2013), Copenhagen (Dano-Norwegian seminar on historical linguistics, January 2014), Tromsø (NORKOG, June 2014), Bergen (Forskergruppe nordisk språkvitenskap, September 2014), and Berlin (Freie Universität, December 2014), in particular to Tore Nesset, Helge Sandøy and Horst Simon. Very helpful comments on written versions have been given by Andrew Carstairs-McCarthy, Oda Røste Odden, Arne Torp and two anonymous PBB referees. Thanks for saving me from a number of embarrassing mistakes. Remaining weaknesses are my own.