California composer Henry Cowell was a pivotal figure among early American modernists and is credited with coining the term "tone cluster" to describe chains of adjacent tones. Cowell introduced the practice of playing directly on piano strings and was a pioneer in composing for percussion and in the study of world music. One of the most prolific composers of the 20th century, Cowell wrote nearly 1,000 works, though about 150 are considered "lost." Afflicted with a learning disorder from childhood, Cowell found it easier to write music rather than words, and required help in authoring three books and numerous articles.