From the introduction: "The main body of this paper is concerned with drawing a parallel between Khmer and Vietnamese. We will endeavor to show the possibility of applying the tongue-root hypothesis to Vietnamese as it is already being applied to Khmer. (This hypothesis explains the grouping of certain phonological features in terms of historic advanced or retracted tongue-root position.) To do this we will be referring to "exponents of advanced (or "retracted" as the case may be) tongue-root position. An exponent is a co-occurring phonological phenomenon which may be considered as dependent on the occurrance of another phenomenon, the independent variable in this case being tongue-root (t.r.) position. [...] This paper uses some of [Henri Maspero's] data to support the hypothesis that t.r. position is of foundational importance in properly describing the development of a two-register tonal system in Vietnamese."
"Vietnamese, register and tongue-root position,"
Work Papers of the Summer Institute of Linguistics, University of North Dakota Session: Vol. 14, Article 7.
Available at: https://commons.und.edu/sil-work-papers/vol14/iss1/7