It is always a little risky to nominate a specific year as the dividing line between one style and another and some overlap will invariably occur. The more linear style of the early masses is still present after 1956, with works like the Missa super “Salve Regina” et “Vater unser im Himmelreich,” whereas the motet Hoc corpus (1951) is already a harbinger of the much later, extremely complex a capella style. Nevertheless, even for just a very noticeable change in Heiller's organ style, 1956 seems right for a very delicate divide.

It is sometimes quite interesting to reflect on which works a composer did not write—partly because of time constraints, or because issues concerning instruments or instrument groupings prevented it. During the years between 1947 and 1957 Heiller was very much inspired by the sound of the human voice, and this smoothed and softened the sharp edges of the harsh linearity of his early organ works and their big intervallic leaps. Of course, we also find leaps, dissonances, and chromaticism in his vocal style, despite the fact that Gregorian chant is an ever-present inspiration and model. And it seems that during this period ideas for another medium did not offer themselves, not even in the case of a concrete project with tempting performance possibilities. In Haarlem, Heiller was approached for a composition for the chamber orchestra (recently taken over by Albert de Klerk). However, in August 1953 he wrote to his mother from a vacation in Vorarlberg, complaining: “Unfortunately, I don't manage to compose at all, and not because there is no time for it, but because I can't think of anything. I have given up completely on that composition for strings for Holland; at best, I could really only write for choir again.”

In April 1957 the Civic Orchestra of Minneapolis got in touch with Heiller and requested that he write a piano concerto for them, “not too difficult.” They offered a commission of $150. Heiller accepted, but added that he could not promise it earlier than autumn 1959. Soon after, in the summer of 1957, another request arrived.