title_body: What is an overview of sound changes? One of my works-in-progress is a language called Proto-Oreadin.  As the name suggests, it is a proto-language, which eventually I am hoping to expand into a family of languages.  Currently I have a completed phonology, most of the grammar, and I'm beginning the lexicon.  My plan to create these daughter languages is by using the zompist.com sound change applier app to evolve the words of my language into multiple daughter languages, and then apply semantic shifts, borrowings, and other pieces of worldbuilding context—but that's a separate issue.
  
  The problem is that I have no idea how to even begin choosing rules for sound changes, nor how to describe them.  I've looked at examples of natural languages, but I'm not sure what's going on with the notation, nor have I been able to identify any patterns that will help me create realistic sound changes for my languages.
  
  
  How are sound changes notated?
  What are the general categories of sound changes?
upvoted_answer: To answer part a), the basic syntax of the notation goes like this:
  
  
    [before] > [after] / [context]
  
  
  The part after the slash gives the situations in which the sound changes occur. For example, the hypothetical spirantization of [b] to [v] intervocalically could be notated as follows:
  
  
    b > v / V__V
  
  
  This rule indicates that b becomes v when surrounded by a vowel (V) on each side. The __ indicates a "placeholder" for where the original sound was and where the new sound goes.
  
  Some other common notations are # for word boundaries and [+feature] or [-feature] to indicate the presence or lack of a certain feature respectively. For example, final-obstruent devoicing in e.g. Russian could be
  
  
    [+stop][+voiced] > [-voiced] / __#
  
  
  where the __# means that the stop in question (__) is at the end of a word (#).
  
  As for b), the best way to get an idea for what kinds of things generally happen in languages is honestly probably just by reading about lots of different languages and seeing what tends to happen. Some of the most common general processes are:
  
  
  Lenition, probably one of the most common sound changes, in which a "softer" sound becomes a "louder" sound. (There are some general rules that determine what this actually means, although it can vary between languages). A few examples are intervocalic flapping in many English dialects (/bɛtər/ → [bɛɾɚ]) and the pronunciation of /b, d, g/ as fricatives or approximants, roughly [β, ð, ɣ], in most contexts in Spanish.
  Assimilation is another frequent sound change. This occurs when features of a given phoneme are influenced by the phonemes around it. For example, in English and Spanish, /n/ becomes [m] before bilabials and [ŋ] before velars (e.g. &lt;rainbow> [ɹ̠ʷejmbow], &lt;concordar> [koŋkoɾðaɾ]).
  Similar ideas are umlaut and vowel harmony, in which vowels change depending on other vowels in the word. Remnants of Germanic umlaut can be seen in English, e.g. foot/feet, and vowel harmony is prominent in e.g. Turkish, where all of the vowels in a word have to harmonize (for the most part) so that affixes have several different forms (for example, the genitive suffix is [in] after front unrounded vowels, [ɯn] after back unrounded vowels, [yn] after front rounded vowels, and [un] after back rounded vowels).
  
  
  While there are general patterns, it's also important to note that some sound changes seem completely illogical and random, and just... sort of happen. The most infamous examples are the ruki rule, where [s] became [ʃ] but only after [r, w, k, j] (?), and strangest of all, Proto-Indo-European *dw to Armenian erk (???). So you really have a lot of room for creativity here, while still being able to be naturalistic.