It was the language of three successive polities in the region, Funan,[4] Chenla and the Khmer Empire (Angkor), which, at its zenith, ruled much of mainland Southeast Asia from the Mekong Delta west to the Andaman Sea and from the Gulf of Thailand north to China.

After the 14th century sack of Angkor by the Siamese Ayutthaya Kingdom, the Khmer Empire was terminally weakened and steadily lost both its hegemony and prestige in the region.

Territory north of the Dangrek mountains was lost to Lao kingdoms while the west and northwest succumbed to the forerunners of the Thais.

The center of Khmer culture retreated southeast and eventually was reduced to a small wedge between its powerful neighbors, Thailand and Vietnam, both of which vied for control of the rump polity as a vassal state.

[8] While this makes understanding Middle Khmer important, it has severely hindered the investigation and reconstruction of Proto-Khmer.

[9] It is assumed that many of the latest texts date to the time of King Ang Duong (1789–1859), to whom is attributed the cbap srei ("Conduct for Ladies").

In addition to inscriptions, there are palm-leaf manuscript from multiple genres or disciplines, including chronicles, romances, ethical treatises and technical manuals.

[15] In addition to the vowel nuclei listed, there were two diphthongs inherited from Old Khmer, [iə] and [uə], and a third, [ɨə], entered the language via loanwords from Thai.

[2][19] Unlike the modern language, Old Khmer contrasted voiced stops /ɡ ɟ d b/ with unvoiced /k c t p/ and had a simple vowels system consisting of eight or nine long monophthongs with short counterparts and two diphthongs.

The secondary characteristic of breathy phonation in the high register became redundant with the development of contrasting vowels for each series and was gradually lost in most dialects by the modern era.

[13] Jenner, based on internal evidence, gives a basic general range between the 16th to 18th century, concluding it wasn't possible to get anymore specific due to the undated nature of most Middle Khmer texts.

In both cases, it is the Old Khmer voiceless stop that is now voiced and their realizations are most commonly implosive [ɓ] and [ɗ], respectively.

Earlier hypotheses assumed Old Khmer, similar to other languages of Southeast Asia, had, in addition to */b/ ~ */p/ ~ */pʰ/ and */d/ ~ */t/ ~ */tʰ/, a fourth series that was both voiced and either implosive ([ɓ], [ɗ]), preglottalized ([ˀb], [ˀd]) or pre-nasalized ([ᵐb], *[ⁿd]).

[29] According to these early hypotheses, since the Indic-based writing system had no symbol for these sounds, the Old Khmer letters for /p/ and /t/ did double duty, also representing this fourth series.

More recently, there have been two theories as to how this seeming "flip-flop" occurred, both of which elegantly integrate the phenomenon into the devoicing shift and attribute the redevelopment of voiced /b/ and /d/ to the complicated phonological details of that process.

According to the authors, this theory best accounts for all the shifts and phonological processes involved as well as explains the current situation in Modern Khmer.

Western Khmer is still between stages four and five; /b/ and /d/ are present, but many vowels have not diphthongized or are in the early stages of diphthongization (i.e. still consist of a monophthong plus a slight glide) and the breathy versus clear phonation contrast is still prominent in most vowels although it is displaying an increasingly lower functional load.