Another way in which the piece is original and innovative is in the instrumentation: pieces for strings, percussion and celesta are rare: I think this is the only one! Bartók creates a unique soundscape with these resources. Another distinctive characteristic of the piece is its division of the strings into two equal groups on opposite sides of the stage with the percussion in between. Bartók specifies this in the frontispiece to the score--again, an early example of a practice that has become more common. The reason he does this is so he can have the two groups echoing one another, an effect common, for example, in 17th century music from Venice where they made use of the layout of one of the churches with its separate choir stalls.
Other significant elements to the piece are the use of a fugal texture for the first movement (which mirrors itself in the second half--for another example see the Third Symphony of Gorecki), the "night music" textures of the third movement and the dance-like character of the second and fourth movements.
Here is the opening theme of the fugue:
It is treated rather exhaustively in fugal style with an exposition of full entries, stretto, fragments of the theme developed, a climax at the most remote point: E flat, a tritone away from the 'home' note of A. After this point Bartók reverses course with the theme in inversion and returns to the beginning. At the very end the structure of the whole movement is presented in microcosm as the violins move from A out to E flat in contrary motion and then return:
The basic pitch content of the piece, which you could summarize like this:
alternates semitones and minor thirds. The inspiration for this may have been various kinds of modal chromaticism found in folk musics of eastern Europe. Modal chromaticism refers to the use of, for example, both B flat and B natural in the above example and C sharp and C natural. Western European music, for which the staff notation was developed, virtually never uses scales in which both chromatically inflected and noninflected notes are used.
I won't go on and analyze the other movements except to point out that an expanded version of the theme from the first movement returns in the last movement to give the overall form a cyclic quality. By "expanded version" I mean that the intervals are all made into wider ones. Here is that version:
I will say that this is one of Bartók's most powerful pieces. Each movement projects a unique atmosphere and mood and the piece as a whole is extraordinarily successful. Let's listen:
This is counted as being a good example of 'progressivism' in 20th century music, but you could equally look at it as an example of a composer looking both back in music history and outside the mainstream of European music for inspiration. Just look at the elements that are historic: fugue form, slow-fast-slow-fast layout of the movements and antiphonal groups. The pitch content is the most 'progressive' element along with the way the percussion is used.