The monastic customaries from the ninth to eleventh centuries --, that is more or less from the moment they first appeared to the time they were mostly replaced by constitutions and statutes --, have usually been conceived by scholars as complementing the Rule of St Benedict (RB). In other words, they believe the RB tell us how these monks and nuns were living, and customaries serve to clarify certain areas of their lives left unclear by the RB. In this paper, I would like to challenge this approach and propose to turn it upside down. I will focus especially but not exclusively on the customaries I know best, the four describing the daily life and liturgy in the abbey of Cluny (Burgundy), written down between the late tenth and late eleventh century. My claim is that we should not start the study of life within a given monastery during the central Middle Ages with the conviction that the regulations of the RB were followed to the letter. Rather, we should use the customaries and other available sources (especially archeology and charters, but also lives of saints) to see which of the regulations of the RB were followed and which were not. Once the constitutions and statutes are in place, scholars have less problems privileging the latter over the RB. The same should be done about customaries even though their modes of production and diffusion were far more complex.