In phonetics, vowel reduction is any of various changes in the acoustic quality of vowels as a result of changes in stress, sonority, duration, loudness, articulation, or position in the word (e.g. for the Creek language), and which are perceived as "weakening". It most often makes the vowels shorter as well. Vowels which have undergone vowel reduction may be called reduced or weak. In contrast, an unreduced vowel may be described as full or strong. Transcription There are several ways to distinguish full and reduced vowels in transcription. Some English dictionaries mark full vowels for secondary stress, so that e.g. is a full unstressed vowel while is a reduced, unstressed schwi. Or the vowel quality may be portrayed as distinct, with reduced vowels centralized, such as full vs reduced or . Since the IPA only supplies letters for two reduced vowels, open and mid , transcribers of languages such as RP English and Russian that have more than these two vary in their choice between an imprecise use of IPA letters such as and , or of custom non-IPA (extended IPA) letters such as and . Weakening of vowel articulation Phonetic reduction most often involves a mid-centralization of the vowel, that is, a reduction in the amount of movement of the tongue in pronouncing the vowel, as with the characteristic change of many unstressed vowels at the ends of English words to something approaching schwa. A well-researched type of reduction is that of the neutralization of acoustic distinctions in unstressed vowels, which occurs in many languages. The most common reduced vowel is schwa. Whereas full vowels are distinguished by height, backness, and roundness, according to , reduced unstressed vowels are largely unconcerned with height or roundness. English , for example, may range phonetically from mid to to open ; English ranges from close , , , to open-mid . The primary distinction is that is further front than , contrasted in the numerous English words ending in unstressed -ia. That is, the jaw, which to a large extent controls vowel height, tends to be relaxed when pronouncing reduced vowels. Similarly, English ranges through and ; although it may be labialized to varying degrees, the lips are relaxed in comparison to , , or . The primary distinction in words like folio is again one of backness. However, the backness distinction is not as great as that of full vowels; reduced