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In linguistics, a surface filter is type of sound change that does not operate on a single set of sounds at a particular point in time, but continues to operate over a longer period. Surface filters normally affect any phonetic combination that is not permitted according to the language's phonetic rules, and therefore exist as a way to preserve the phonotactics of that language. They are also often a source of complementary distribution between certain sets of sounds.
A trivial example of a surface filter is the replacement of sounds that are foreign to the language with sounds that are native to the language they are borrowed into. For example, a language that has no front rounded vowels may replace such vowels with either front unrounded or back rounded vowels, whenever it borrows a word containing such a vowel. Strictly speaking this is not a surface filter, since it is merely the way in which the phonetics of one language are matched to that of another. But it does illustrate the importance of surface filters in preserving the phonological structure of words within the language. Usually, the term "surface filter" applies only to rules that affect natively constructed words as well as borrowings.
One very common example of a surface filter is final-obstruent devoicing, in which voiced obstruents that occur at the end of a word are automatically converted to their unvoiced counterpart. Such a sound change is not a regular sound change. If it were, the devoicing would occur only at a fixed point in time, and any new words that enter the language at a later stage might end in voiced obstruents after all. But this does not happen; any new words are automatically "passed through the filter" and their final obstruents are devoiced automatically. This happens even as a result of apocope of final vowels, which causes non-final obstruents to become final. A historical example in Dutch occurs in many verbs, such as blazen ("to blow"). The original Middle Dutch first-person singular form was blaze, but when the final -e was lost, the form did not become *blaaz (doubling of the a being merely a spelling convention); the now-final -z was automatically devoiced to create the modern form blaas.
Two other examples of surface filters, which occurred in the history of the Germanic languages, are Sievers' law and the Germanic spirant law. In the former, there was a restriction on the distribution between -j- and -ij-, so that -j- could only appear after a consonant following a short vowel, and -ij- would occur anywhere else. This process was automatic, and affected newly created words and loanwords as well: the three-syllable Latin word puteus ("pit, well") for example was borrowed into Germanic as the two-syllable *putjaz, because the more faithful rendering *putijaz was not permitted by the phonological rules dictated by Sievers' law.
The Germanic spirant law, similarly, affected combinations of an obstruent followed by -t-. According to the formulation of the law, such obstruents were automatically converted into fricatives (with dentals becoming -s-) and devoiced. Again, a loanword from Latin exemplifies this: Latin *scriptum was borrowed into Germanic as *skriftiz, with the disallowed combination -pt- being replaced by -ft-.
Surface filters are often formed as a result of sound changes that change the phonetic makeup in a way so that certain sounds or combinations no longer occur in the language. As a consequence of this, speakers no longer learn to pronounce these combinations and will therefore have difficulty with new words that violate these principles. At this point, two things can happen: either the phonology of the language is extended to incorporate such new combinations, or the "inconvenient" combinations are automatically reconstructed into a form that conforms to the phonotactics of the language. If this reconstruction occurs systematically and becomes part of the phonology of the language, the result is a surface filter.
Such phonological rules may continue to apply for an indefinite amount of time. Final-obstruent devoicing in Dutch, for example was a phonological rule in the language already since the Old Dutch period over a thousand years ago. The Germanic spirant law may have been formed as part of Grimm's law long before written records began, but it ceased to operate shortly after the Germanic languages began to break up, around the middle of the first millennium.
Sometimes, sound changes occur that directly violate a surface filter, and this may cause it to cease operating. Sievers' law presumably lost relevance in the West Germanic languages after the operation of the West Germanic gemination, since it eliminated the contrast between light and heavy syllables that was at the core of the law's operation.