Bi-tonality: Composing with new harmonic movement
At the turn of the 20th Century, a number of composers were beginning to explore the sound world of bitonality, the use of more than one key simultaneously. Composers like, Bartók, Stravinski and Ives all explored bitonality in their music. Debussy's second book of Preludes explores the blending of keys - as does my own set of 12 Preludes.
Some theorists consider notation in two keys to be pointless, because the purpose of a key is to suggest a root or tonic. With bitonality, there is no common tonic, and so the keys become irrelevant. And yet surely the point of a key signature is to also aid in the reading/performing of the music?!?
The key signatures return for the fifth prelude with the keys of A-flat and E. Yet, in the sixth prelude I opted to write it in the key of C-sharp (7 sharps), then flatten or sharpen the necessary notes to create the keys of B and D-flat (enharmonically). The seventh prelude is written in 6 sharps as F-sharp and G-flat are the same key on the piano - so in many respects I had returned to what the first prelude experienced, mono-tonality the seventh ends in the relative minor whereas the first prelude ends in major.
What I discovered in the process was sometimes key signatures are a hindrance to the performance of a piece. Pretty much every pianist who has looked at the preludes suggests they should be written without a key and just place accidentals where necessary. However, I feel by putting in the key signatures I am encouraging the pianist to "think" in multiple keys, which - if pianists ever get to the point they are truly thinking that way, the music they will conceive will be vastly different than what we experience today; they will discovered a blending of the keys we don't understand.
This is rather like someone learning another language. The second language is difficult to learn, the third not so much and from there learning another language just gets easier as grasping linguistic concepts across multiple languages gets easier with more languages to draw on.
When working with multiple keys, I found it important to understand the common points. Even with keys as remote as A and D-flat (fourth prelude) there are points where the two keys intersect. There intersections then become the focal point for propelling the composition forward, although they may or may not end up being the "tonic" or resolution notes.
Then it is important to look at the complex scale, the meta-scale of both keys combined. What is the mode presented with the meta-scale? In the case of the second prelude, D is the relative minor for F and the dominant for G, so an easy key to migrate toward in terms of working with these two keys. I would say playing meta-scale and listening to it again and again to gain a sense of its migration might work, however, this tends to point toward the use of pre-conceived tonal leanings and not necessarily allow for something new. This is the process I used with the first few preludes (up through prelude 6) and why I feel these early preludes are not as strong as the later ones - where I took a more analytical approach to determining the key note for the piece.
Some of the preludes move harmonically similar to mono-tonal piece in that chord structures can be analysed showing a IV, V, I progression. However, other preludes tend to move harmonically linearly, moving the harmonies up or down along the scale. Still other preludes are grounded in a tonal center, shifting the tonal centre at times, to create harmonic movement, but not in a typical cadence type manner.
As tonality moves forward, I think there will be more exploration of this sort of bitonality and the resulting harmonies. Perhaps we will not need to express the music in terms of a written key (as we know it today) - but bitonality can create vastly definitely different harmonic movement from standard tonal progressions - and thus opens a whole new world to possibilities.
While the project of writing these preludes helped me discovered some interesting aspects of bitonality, and perhaps a better understanding as to what happen harmonically when two keys merge. However, looking back on these pieces (after more than a year) I realise they aren't as successful at achieving a true sense of bi-tonal writing. Part of the problem is the way I approached determining the two keys. I think, if I had a method of determining keys that wouldn't eventually be the same key enharmonically the resulting pieces (at least the first and seventh) would have been bi-tonal.
It is interesting to me that Prelude 4 and Prelude 10 are in the same keys (with the hands reversed) and yet, Prelude 4 centers around D while Prelude 10 centers around B (it was by choice they are different).