1. The comparative-historical method in linguistics

2. Phonetic Law. Types of the sound changes.
In linguistics, regular changes that occur during the development of a language in its sound structure (compare the disappearance of the vowels 2 and b in Russian) or in the phonetic structure of words (compare the replacement of e by o in Russian; for example the present-day nes [nyos] “he carried” from the old nes [nyes]). Spontaneous and combinative changes are distinguished from each other; the former occur in all instances where a corresponding sound is encountered; for example, in Russian?? has been replaced in all positions by the vowel e. The second type of change occurs only in certain phonetic positions; thus, the replacement of e by o took place only in the stressed syllable, if a hard consonant followed the e (compare podenii, “daily,” with den’, “day”). With the development of phonology came historical, or diachronic, phonology, in which phonetic laws were examined in the aspect of the study of the phoneme.
Sound change includes any processes of language change that affect pronunciation (phonetic change) or sound system structures (phonological change). Sound changes can be environmentally conditioned, meaning that the change in question only occurs in a defined sound environment, whereas in other environments the same speech sound is not affected by the change. The term "sound change" refers to diachronic changes, or changes in a language's underlying sound system over time; "alternation", on the other hand, refers to surface changes that happen synchronically and do not change the language's underlying system (for example, the -s in the English plural can be pronounced differently depending on what sound it follows; this is a form of alternation, rather than sound change). However, since "sound change" can refer to the historical introduction of an alternation (such as post-vocalic /k/ in Tuscan, once [k], but now [h]), the label is inherently imprecise and often must be clarified as referring to phonetic change or restructuring.
Sound change is usually assumed to be regular, which means that it is expected to apply mechanically whenever its structural conditions are met, irrespective of any non-phonological factors (such as the meaning of the words affected). On the other hand, sound changes can sometimes be sporadic, affecting only one particular word or a few words, without any seeming regularity.
For regular sound changes, the term sound law is sometimes still used. This term was introduced by the Neogrammarian school in the 19th century and is commonly applied to some historically important sound changes, such as Grimm's law. While real-world sound changes often admit exceptions (for a variety of known reasons, and sometimes without one), the expectation of their regularity or "exceptionlessness" is of great heuristic value, since it allows historical linguists to define the notion of regular correspondence (see: comparative method).
Each sound change is limited in space and time. This means it functions within a specified area (within certain dialects) and during a specified period of time. For these reasons, some scholars avoid using the term "sound law" — reasoning that a law should not have spatial and temporal limitations — replacing the term with phonetic rule.
Sound change has no memory: Sound change does not discriminate between the sources of a sound. If a previous sound change causes X,Y > Y (features X and Y merge as Y), a new one cannot affect only an original X.
Sound change ignores grammar: A sound change can only have phonological constraints, like X > Z in unstressed syllables. The only exception to this is that a sound change may or may not recognise word boundaries, even when they are not indicated by prosodic clues.
Sound change is exceptionless: if a sound change can happen at a place, it will. It affects all sounds that meet the criteria for change.