A rhythmic surface is underdetermined if there are more beats than there are onsets of actual notes in the measure. Similarly, a rhythmic surface is overdetermined if there are more onsets of actual notes than beats in the measure, such that every note in the measure does not correspond to a beat in the measure (London 2004).
When first presented with a piece of music, listeners must use the initial structural and rhythmic properties of the music to build their metric representation of the piece. Listeners search for invariances and well-known musical figures in the music to inform their understanding of the piece’s meter (London, 2012: 67). The listener does not always need to entrain to every possible level of subdivision (i.e. periodicity) in a piece, however. Sometimes there are too many “surface articulations” (physically sounded rhythmic events) for all to be considered as corresponding to a beat location; in this way, a piece could be metrically overdetermined and the listener will need to determine what information to filter out (London, 2012: 73, 76). On the other hand, if there are fewer surface articulations than beat articulations, the piece is said to be metrically underdetermined, and the listener will have to internally generate the tactus using the limited information given by the music as a basis (London, 2012: 75).