title_body: How did Tolkien model diachronic changes in his Elvish languages? Wikipedia explains that Tolkien modelled changes to his Elvish languages, something we'd call diachronic conlanging these days:
  
  
    Tolkien conceived a family tree of Elvish languages, all descending from a common ancestor called Primitive Quendian. He worked extensively on how the languages diverged from Primitive Quendian over time, in phonology and grammar, in imitation of the development of real language families.
  
  
  What is a summary of the major changes he modelled? For example, did he describe something akin to the Great Vowel Shift?
upvoted_answer: OK, here are some details on classical Quenya and classical Sindarin (based on Helmut W. Pesch, Das große Elbisch-Buch, Bastei-Lübbe 2009)
  
  The phoneme inventory of Primitive Quendian was
  
  p   t   k
  pʰ  tʰ  kʰ
  b   d   g
  m   n   ŋ
      r,l
          ɣ
  w     j
  a, e, i, o, u, ai, oi, ui, au, eu, iu
  
  
  An early addition to this repertoire were prenasalised consonants mb, nd, ŋg that developped differently in Quenya and Sindarin.
  
  Sound shifts for Quenya
  
  pʰ   tʰ   kʰ  -&gt; f   θ   x  (later: θ -&gt; s)
  b-   d-   g-  -&gt; v-  l-  Ø
  -b-  -d-  -g- -&gt; -mb- -nd- -ŋg-
  mb-  nd-  ŋg- -&gt; m-  n-  ŋ-   (later: ŋ- -&gt; n-)
  
  
  The loss of almost all b's, d's, and g's makes Quenya phonological similar to the Finnish language.
  
  Quenya has some old compounds that preserve the pre-shifted state of a root, e.g., for dome > lome "night, darkness" there is an old compund tindome and a newer regular one Aldalome. This adds to the diachronical depth of the language.
  
  Sound shifts for Sindarin
  
  p    t    k   -&gt; b   d   g
  pʰ   tʰ   kʰ  -&gt; v   ð   ɣ (later: ɣ -&gt; Ø)
  mb-  nd-  ŋg- -&gt; b-  d-  g- 
  kʷ  -&gt; p
  w-  -&gt; gw-
  
  
  Sindarin also has a lot of lenitations (i.e., in certain contexts the initial consonsonant of a word is changed to some "softer" consonant) inspired by Welsh. The lenitations are different for original b d g and b d g that come from mb  nd  ŋg. Sindarin also has acquired an umlaut y. 
  
  EDIT 3: Evolution of initial sp-, st-, and sk-
  
  Both in Sindarin and quenya, consonant clusters with an initial s were simplified in two steps: First, the stop became aspirated and the s was dropped
  
   sp st sk -&gt; pʰ tʰ kʰ
  
  
  and than the aspirated stops became voiceless fricatives
  
   pʰ tʰ kʰ -&gt; f θ h
  
  
  In Quenya, also the so-created θ's became s later, such that the net development of Quenya is
  
    st -&gt; s
  
  
  EDIT 1: Some more information gleened from the dictionary
  
  Quenya Words beginning with h- derive from Primitive Quendian roots beginning in kʰ-. The initial consonant cluster stʰ was simplified via θ to s.
  
  Some words beginning with  ç- like hyarmen "south; left side" come from roots beginning in kʰj.
  
  Sindarin  has simplified some consonant clusters like kl- kr- via intermediate voiceless l and r but in classical Sindarin those l's and r's are the usual voiced ones (despite being retained in orthography as lh rh).
  
  EDIT 2: The grammatical changes aren't explained in great detail. It seems that Tolkien applied some handwavium here when he states that Quenya was consciously changed for greater clarity. On the other hand, Sindarin has evolved naturally (in Tolkiens internal history) in the woodlands of Middle Earth where everything is interwoven with everything.