A "rāga" is a central concept of Indian music, predominant in its expression, yet the concept has no direct Western translation. According to Walter Kaufmann, though a remarkable and prominent feature of Indian music, a definition of "rāga" cannot be offered in one or two sentences. "rāga" is a fusion of technical and ideational ideas found in music, and may be roughly described as a musical entity that includes note intonation, relative duration and order, in a manner similar to how words flexibly form phrases to create an atmosphere of expression. In some cases, certain rules are considered obligatory, in others optional. The "rāga" allows flexibility, where the artist may rely on simple expression, or may add ornamentations yet express the same essential message but evoke a different intensity of mood.