Already in some of Bartók's juvenile compositions, a definite attraction is manifested for the irregular rhythmics. This is an early appearance of
metrics, opposed to the conventional
one. An extremely rich selection for irregular meters was offered to Bartók by Rumanian folk song material, particularly in the “colindas”. Speaking about, although Bartók used the term “Bulgarian rhythm”, although knew well this was not an exclusively Bulgarian peculiarity. The phenomenon shows clearly the existing form of the additive-substractive metrics, because the quantitatively various bar structures alternate by augmenting or diminishing by one quaver (as common nominator), a kind of “mistuning in the time”. This thinking has been well documented in a series of various examples, but the
Sonata for two pianos and percussion
(1937) provides an extremely rich selection of rhythm combination, in which the inner overstructurating of the bar, resp. the mistuning of the meter plays the principal role (3+3+3/8à1+2+2+2+2/8à 4+2+3/8).