Historical Linguistics/Sound Change
If you have ever studied a language related to one you already known well, you might that certain sounds seem to consistently appear in one language where others appear in the other. For example, if you have studied Latin as well as English, you might observe that words which begin with a "p" in Latin often have equivalents in English beginning in "f". For example:
This pattern is not coincidental. English and Latin are both descended from an ancient language of an unknown name, which linguists today call Proto-Indo-European, often abbreviated to PIE. The Germanic descendants of PIE all underwent a sound change called Grimm's Law in which [p] became [f] in a process known as Grimm's law. Latin is not a Germanic language, and consequently maintains the [p] sound in many words where it was changed to [f] in English.
We can write this change in the following notation: p > f
Historical linguistics is heavily concerned with these sound changes, as they can reveal a lot of information about what languages were like in the past.
The Neogrammarian HypothesisEdit
A critical feature of sound change is that it is almost always uniform across a language. Sound changes, critically, do not occur slowly on a word by word basis, but rather seem to occur to every word matching the conditioning environment of the change in a period of one or two generations. Apparent exceptions are often words introduced to the language after the sound change has occured, preventing them from being affected.
Not all sound changes occur to every appearance of a particular sound. Instead, they can occur to every sound in a particular environment. For example, between Very Old Latin (a term actually used by linguists) and Classical latin, [s] became [r] between vowels. We not the a change occured in a particular environment with the character '/'. The above example might thus be notated: s > r / V_V. the part after the '/' indicated the conditioning environment. The underscore '_' represents the sound undergoing the change, whereas the Vs represent vowels.
Sound changes are historical events: they happen at a point in time, and in a specific order. The order is often extremely important, as different rule ordering can produce different results. For example, in addition to the above rule (s>r / V_V), Very Old Latin also underwent another change, ss>s / _Vː (long Ss become short before long vowels). Consequently, there exist words in Classical Latin which seem to violate the first rule, such as "causa" (cause). However, at the time the change occurred, the word was caussaː, meaning that the original rule, which only applied to short Ss, did not apply at the point in history in which it happened.