This selection is usually titled as being in A minor; however, the work is really centered around the note 'A'. There is no typical harmonic structure to suggest a tonality of A-minor. There is, however, a repetitive use of the note 'A' around which many of the musical ideas are based. There are tonal passages as well as bi-tonal passages (two key areas at once). Bartok's research into his native folk music also affects the modal harmonies which appear throughout all of the movements. 

The second movement is in rondo form and is very dissonant. The material is highly developmental making theme recognition difficult. The rhythmic activity generates an energetic texture. Notice the rich and contrasting sonorities. They are a product of stark dissonances and ingenious scoring. Although the movement is marked "allegro" it fluctuates in tempo. The texture shifts dramatically; activity levels change making it sound as if the tempo has slowed. Notice how difficult it is to anticipate the rhythmic pulse. See if you can find metric groupings. 