Sentence:  Different levels of <WSD>mensuration</WSD> have different roles in the musical structure. The most prominent one is the one corresponding to the compositional tactus. Rhythmic groups may contradict the <WSD>mensuration</WSD> temporarily, but in the examples in this book, they never do so throughout an entire piece. 
Meaning of the word marked with <WSD> tags:  A 13th century system for governing rhythmic relationships in music that was a precursor to the modern use of time signatures; The use of (mensural notation).