The first part of this article describes the compositional process in Nielsen’s String Quartet Op. 5, on the basis of an examination of the sketches, additions and deletions found in the draft score. In the majority of cases the sketches are short and closely related to the musical material beside which they are notated. Close examination shows that Nielsen evidently had a sufficiency of ideas but that he often had problems working them into context. Apart from the short sketches there are three versions of a central section in the second movement. On the basis of an analysis of the entire movement – including the motivic working in the various middle sections and their relationship to the outer sections – the second part of the article gives a rationale for Nielsen’s third and final version.