After giving an overview of various analyses of Density 21.5, this article compares several segmentations in order to show how the links made between this work for solo flute and the history of musical language affect these segmentations. It will be seen that the different concepts of what makes a motive and the structural importance given to them are the first example of what affects the segmentation. The foregoing observations open the discussion on the independance between the musical parameters (melodic line, motive, intervals) in Density 21.5, which can be seen through the comparison of the segmentations into two or three large parts. In turn, this independance helps to explain the ambiguity and tension at the beginning of the piece, which is highlighted by the comparison of segmentations on a smaller scale.