Nielsen studied Renaissance polyphony closely, which accounts for some of the melodic and harmonic content of his music. This interest is exemplified in his "Tre Motetter" (Three Motets, Op. 55). To non-Danish critics, Nielsen's music initially had a neo-classical sound but became increasingly modern as he developed his own approach to what the writer and composer Robert Simpson called progressive tonality, moving from one key to another. Typically, Nielsen's music might end in a different key from that of its commencement, sometimes as the outcome of a struggle as in his symphonies. There is debate as to how much such elements owe to his folk music activities. Some critics have referred to his rhythms, his use of acciaccaturas or appoggiaturas, or his frequent use of a flattened seventh and minor third in his works, as being typically Danish. The composer himself wrote "The intervals, as I see it, are the elements which first arouse a deeper interest in music ... t is intervals which surprise and delight us anew every time we hear the cuckoo in spring. Its appeal would be less if its call were all on one note."