Uniting the most talented Dutch improvisers from several generations Instant Composers Pool Orchestra (ICPO) is marking 50 years of its existence this year. To hear this glorious collective playing in Vilnius is a unique event also because along with other coryphées on stage the audience will meet the legendary drummer Han Bennink, celebrating his 75th anniversary this year.

Misha Mengelberg, a Fluxus art movement zealot, and the celebrated jazz pianist and composer who past away earlier this year, and Bennink founded the ICPO in 1967. For half a century Mengelberg was the spiritus movens of the Orchestra. The great jazz anarchist Willem Breuker who appeared in Vilnius Jazz with his legendary Willem Breuker Kollektief also was the witness of the arrival of the ICPO.

Mengelberg and Bennink have played together since 1961; before long they’d played on Eric Dolphy’s 1964 Last Date and in a successful Dutch quartet, until they brought in the rebellious saxophonist Willem Breuker, whose disruptive presence tore the group apart. However, the three musicians collaborated in Instant Composers Pool until 1974 when Breuker left to form his own Willem Breuker Kollektief, a flagship of Dutch improvised music.

It took time for the band to mature into something very like the present-day ICPO. When Breuker left, Mengelberg and Bennink founded the raggedy ICP Tentet featuring German saxophonist Peter Brötzmann and sometimes cellist Tristan Honsinger. In the 80s younger players joined, some of whom have been there ever since: trombonist Wolter Wierbos, saxophonists/clarinettists Michael Moore and Ab Baars and bassist Ernst Glerum.

The ICPO flourished in the 90s when German trumpeter (and now New Yorker) Thomas Heberer, new music violinist/violist Mary Oliver and saxophonist/clarinettist Tobias Delius joined. Cellist Honsinger re-joined in the 90s as well.

So the ICPO is a mixed ensemble: part jazz band, part chamber orchestra. On stage, the space between piano and drums is occupied by a thorny string trio: viola, cello and double bass. Across from them is a five-piece horn section, three reeds and two brass. Nothing else and no one else sounds quite like them and no two gigs are ever alike. They generate a remarkable emotional spectre ranging from sweetly lyrical to sarcastically sour.

Mengelberg had studied composition at the Hague conservatory. In his 60s game piece Hello Windyboys two wind quintets variously engage in call and response, communicate in musical code, interrupt or block each other, or seduce their rivals into cooperating. They did formally what ICPO’s musicians now do informally. Under Mengelberg’s original strategies the musicians learned how to deal with fellow players’ quirks of timing or intonation, how to confound their colleagues before their colleagues could confound them, how to bend or subvert the music in performance, and run little subroutines within a piece.

To keep things fresh, Mengelberg would write out and distribute set lists moments before the players hit the stage. Those lists typically included several compositions separated by spontaneous improvised sub-groups, allowing the players the option of improvising their way out of and into the written themes. Nowadays, all the players take turns writing those set lists.

Now pianist Guus Janssen replaced the leader. Today the ICPO’s programmes feature new and old tunes, as well as new arrangements of older Mengelberg tunes.