Starting from a recorded improvisation in a nineteenth-century idiom (sound file 1), the idea is developed that a tonal improvisation can be seen as being driven by guiding principles in the form of musical conventions which can be understood as elements of a musical language. For these conventions, musical ‘commonplaces’, the rhetorical term loci communes is proposed. Musical loci, it is argued, play an important role in music possessing a certain amount of predictability – and in music being ‘understandable’ at all. Some categories of loci, like those related to form, melody and harmony, show a remarkable resemblance to music-theoretical concepts. A crucial difference however is that, unlike loci communes, concepts in the teaching of music theory have a tendency to become a thing, an object, that can be written down and labelled. A musical event in this way becomes a ›cadence‹, a ›ternary form‹, a ›motivic connection‹. This process of concepts becoming objects can be termed reification: a musical gesture becomes a res, a thing, without the temporal dimension of sounding music. For an improviser however this temporal dimension is of crucial importance, as a result of which music theoretical concepts have only limited value. The phenomenon of improvisation can put music theory as a conservatory subject in a different light. There are good reasons to stress and develop the generative aspect that music theory also can have. The concept locus communis, as proposed here, might be a useful tool to describe the process.