Proto-Halfling language

Proto-Halfling was the ancestor of the Halfling languages. Since the Halfling language is quite conservative in its grammar, and hasn't had too many sound shifts in the last millenium, it is difficult to exactly pinpoint where does Proto-Halfling end and a form of Halfling that could be described as "close enough to the contemporary" begin, although the definite point of transition is usually put at the 9th century BEKE, when the proto-language fragmented into clearly recognizeable dialects that still remain mutually intelligible to this day.

Earlier developments
When Proto-Halfling and Old Dwarven weren't developed yet, but were instead still diverging dialects of Proto-Norlokian, some developments - especially in the articulation of vowels - were already starting to set the two future languages apart:

These early vowel shifts ensured that in Halfling, the front vowels  would only appear after slender consonants and the back vowels  would only appear after broad consonants. Only the neutral schwa was allowed to appear after both broad and slender consnants.

Developments in Early Proto-Halfling

 * The labio-palatal unrounded to.
 * The distinction between the sibilant fricatives and affricates  shifted towards a distinction between postalveolar  and alveolar . They would be realized as affricates word-initially, after nasals and when geminated, fricatives otherwise.
 * All of the plosives developed lenited allophones  in postvocalic positions, but only when ungeminated.
 * The flaps became trills  when word-initial or geminated.
 * The schwa became silent, unless it would have created a consonant cluster larger than three consonants (or two word-initially), or word-initial plosive+plosive, nasal+plosive, nasal+sonorant, sonorant+plosive clusters. In other words, word-initially, only sibilant+plosive, sibilant+sonorant and plosive+sonorant clusters were allowed word-initially.
 * In contaxts where didn't become silent, it became  after slender consonants,  after broad consonants.  was always retained if it was part of a diphthong.
 * The glottal stop became silent. If it formed a consonant cluster with any other consonant, it put compensatory lengthening on it.
 * Vowel shifts:
 * Shifts conditioned by broad consonants: →
 * This shift gave the unique ability of being able to appear between broad consonants and front vowels. In loanwords,  would be prosthetically inserted between broad consonants and front vowels of any kind.
 * Shifts conditioned by slender consonants: →
 * Before this shift, only front vowels were allowed to appear after slender consonants, but this shift demolished that short-lived rule.

Early Proto-Halfling

 * was - and still is - a special kind of broad consonant that, unlike other broad consonants, can be followed by both front and back vowels. It also served as a prosthetic consonant between broad consonants and front vowels.

The Transition
The consonantal system of Early Proto-Halfling was not very stable. One of its more fragile elements was the allophony between plosives and fricatives, especially sibilant affricates and their fricative counterparts.

First, the sibilant affricates lenited to regular fricatives before any consonant - even in word-initial clusters. Inconsistent and unsystematic removal of word-initial short vowels in some very commonly used words, as well as prolonged contact with languages that contained word-initial fricatives: namnely, Archaic High Elven and Proto-Human. This cemented the status of those fricatives as separate phonemes from their plosive counterparts. Their voiced counterparts never became separate phonemes.

Late Proto-Halfling

 * was - and still is - a special kind of broad consonant that, unlike other broad consonants, can be followed by both front and back vowels. It also served as a prosthetic consonant between broad consonants and front vowels.

Later developments
All variants of Halfling eventually lost the voiceless dental fricates, but Eastern Halfling variants would eventually regain them, mainly from loanwords from Old Etrandish and Dwarven. Nevertheless, before their reintroduction to Eastern Halfling, this happened to the voicelss dental fricatives in Halfling, depending on the dialect: Unlike Eastern Halfling, Western Halfling would never regain the voiceless dental fricatives, because rather than Dwarven and Etrandish, it was instead heavily influenced by High Elven, which lacked dental fricatives.
 * Th-stopping: →
 * Th-glottalization: →
 * Optionally also →
 * Th-debuccalization: →
 * Optionally also →
 * Th-dorsalization: →
 * Because could also be interpreted as, this could be considered a variant of Th-debuccalization.
 * Th-silencing: →
 * This usually happened with an intermediary stage of (Th-glottalization) or  (Th-debuccalization)

All variants of Halfling eventually merged the trills into one consonantal phoneme that was classified as broad - making it a special kind of broad consonant like, appearing before both back and front vowels alike -, and shifted it to uvular.

Vowels
Unlike the consonantal system of Proto-Halfling, the vowel system was rather stable and didn't really change during the course of the language.

Diphthongs

 * The diphthongs appeared only in loanwords.

Later developments
All variants of Halfling eventually evolved a set of nasal vowels from conflating vowel + nasal clusters, if the nasal wasn't directly followed by a vowel.