The two musical cultures of Thailand and Java, especially as conveyed by the leading percussion ensembles (the Thai pi-phat and the Javanese gamelan), share several important concepts, as well as obvious physical similarities between the instruments themselves. Studies by Becker (1968, 1980) draw attention to the “musical family” of Southeast Asia, especially to similarities within “bronze gong cultures” in that region (Becker 1980: 453). One of her papers is a comparison of Thai and Javanese music (Becker 1980). Another feature that paper shares with the present one is that the stimulus for it came from working with a graduate student whose research lay in Thai music, in the latter case an expert Thai performing artist. The present paper investigates what is perhaps the most striking, and paradoxical, similarity between Thai and Javanese music: the processes of improvisation and elaboration turn out to be governed by a melody which is not actually performed. In both traditions the art of playing individual instruments is, broadly, the art of realising this elusive melody according to the idiomatic patterns associated with a particular instrument. How it is done varies greatly from one tradition to the other, and within each ensemble. Theoretical constraints must also be considered. In both traditions rules must be observed, the purpose of which is to create structures which are shaped and balanced in ways not only conveying aesthetic appeal but also suggesting more universal ideas about melodic organisation and logic. Thai musicians tend to talk about what they do in ways which relate to improvisation (kan phrae tamnong, literally “changing the melody”). A similar process may be observed in Javanese music, even if the notion of improvisation is not as prevalent. The comparable term here is gam-pan (literally “working on”). There is no need to give detailed examples of how kan phrae tamnong and garapan work in specific pieces, as this aspect has been thoroughly documented in the literature of the two traditions—for example Brinner (1995), Martopangrawit (1984), Ketukaenchan (1989), Sorrell (2000), Sumrongthong (1997), and Sutton (1979). The main purpose here is to compare Thai and Javanese concepts of unheard melody and their musical realizations.