In music, a cloud is a sound mass consisting of statistical clouds of microsounds and characterized first by the set of elements used in the texture, secondly density, including rhythmic and pitch density. Clouds may include ambiguity of rhythmic foreground and background or rhythmic hierarchy. Examples include: * Iannis Xenakis's Concret PH (1958), Bohor I (1962), Persepolis (1971), and many of his pieces for traditional instruments * György Ligeti's Clocks and Clouds (1972–3) * La Monte Young's The Well Tuned Piano * Bernard Parmegiani's De natura sonorum (1975)