Tone clusters have generally been thought of as dissonant musical textures, and even defined as such. As noted by Alan Belkin, however, instrumental timbre can have a significant impact on their effect: "Clusters are quite aggressive on the organ, but soften enormously when played by strings (possibly because slight, continuous fluctuations of pitch in the latter provide some inner mobility)." In his first published work on the topic, Henry Cowell observed that a tone cluster is "more pleasing" and "acceptable to the ear if its outer limits form a consonant interval." Cowell explains, "the natural spacing of so-called dissonances is as seconds, as in the overtone series, rather than sevenths and ninths...Groups spaced in seconds may be made to sound euphonious, particularly if played in conjunction with fundamental chord notes taken from lower in the same overtone series. Blends them together and explains them to the ear." Tone clusters have also been considered noise. As Mauricio Kagel says, "clusters have generally been used as a kind of anti-harmony, as a transition between sound and noise." Tone clusters thus also lend themselves to use in a percussive manner. Historically, they were sometimes discussed with a hint of disdain. One 1969 textbook defines the tone cluster as "an extra-harmonic clump of notes."