|It is a long-standing and widely accepted theory that Thai classical music (phleng Thai doem) is based on a tempered tuning system that divides the octave into seven proportionally equal intervals of 171.429 cents, sometimes called “7-tet.” This theory was formulated in 1885 by Alexander J. Ellis, who made observations of fixed-pitch percussion instruments associated with the piphat ensemble. Although the theory is not corroborated by reliable data, it has not been seriously challenged. This paper identifies flaws with this theory and argues that it is incorrect to claim that Thai music is based on theoretical or realized equidistance. In questioning the theory, I consider a range of factors that influence Thai tuning, including the limitations of the tuning methods of fixed-pitch percussion instruments, the aims of those who tune and play the instruments and the non-harmonic proportions of the octave interval on these instruments. I discuss the failure of the equidistance theory to account for the tuning concepts and practices of vocalists and players of non-fixed-pitch instruments and the influence of piphat-associated practices regarding the development and circulation of musical ideas. I show that the theory of equidistance has historically been supported by scholars who have taken the unwarranted step of presupposing that the theory applies equally to all instruments, and I argue that the theory is sustained by data obtained through the use of suspect numerical testing methodologies. I conclude that the theory of equidistant tuning should be abandoned for lack of evidence to support it and incompleteness in its scope. A theory of Thai tuning must aim to accommodate the diverse reality of Thai musical practices and concepts rather attempt to reduce the explanation of Thai tuning to numerical terms.