Per is a serie of 4 pieces: 1 prelude, 2 interludes and 1 postlude. This work was commissioned by Kody festival in 2018. The aim was to put original Ryterski's music in-between the already chosen orchestral pieces (Gyorgy Ligetti's Ramification, Sofia Gubaidulina's Seven words and Arvo Part's Fratres). Originally Per was meant to be inseparable from the rest of the program, especially that at the beginning it was a 4-channel music however, composer decided to revise it and give it a new life.

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Per (in latin per means through) is a piece about movement - from one surround situation to another one, and so on. The movement is designed in a form of rotating 2 lines - from horizontally parallel to crossing each other, and then to vertically parallel.