Some approximants resemble vowels in acoustic and articulatory properties and the terms
semivowel and
glide are often used for these non-syllabic vowel-like segments. The correlation between semivowels and vowels is strong enough that cross-language differences between semivowels correspond with the differences between their related vowels. Vowels and their corresponding semivowels alternate in many languages depending on the phonological environment, or for grammatical reasons, as is the case with
Indo-European ablaut. Similarly, languages often avoid configurations where a semivowel precedes its corresponding vowel. A number of phoneticians distinguish between semivowels and approximants by their location in a syllable. Although he uses the terms interchangeably, remarks that, for example, the final glides of English and differ from French ('through') and ('tub') in that, in the latter pair, the approximants appear in the
syllable coda, whereas, in the former, they appear in the
syllable nucleus. This means that opaque (if not minimal) contrasts can occur in languages like
Italian (with the i-like sound of
piede 'foot', appearing in the nucleus: , and that of 'plane', appearing in the syllable onset: ) and Spanish (with a near minimal pair being 'abject' and 'opened'). : : Because of the articulatory complexities of the American English rhotic, there is some variation in its phonetic description. A transcription with the IPA character for an
alveolar approximant () is common, though the sound is more
postalveolar. Actual retroflexion may occur as well and both occur as variations of the same sound. However, makes a distinction between the vowels of American English (which he calls "rhotacized") and vowels with "retroflexion" such as those that appear in
Badaga; , on the other hand, labels both as
r-colored and notes that both have a lowered third
formant. : Because the vowels are articulated with spread lips, spreading is implied for their approximant analogues, . However, these sounds generally have little or no lip-spreading. The fricative letters with a lowering diacritic, (formerly ), may therefore be justified for a neutral articulation between spread and rounded . In articulation and often diachronically,
palatal approximants correspond to
front vowels,
velar approximants to
back vowels, and labialized approximants to
rounded vowels. In American English, the
rhotic approximant corresponds to the rhotic vowel. This can create alternations (as shown in the above table). In addition to alternations, glides can be inserted to the left or the right of their corresponding vowels when they occur next to a hiatus. For example, in
Ukrainian, medial triggers the formation of an inserted that acts as a syllable onset so that when the affix is added to ('football') to make 'football player', it is pronounced , but ('
Maoist'), with the same affix, is pronounced with a glide.
Dutch for many speakers has a similar process that extends to mid vowels: • → ('cinema') • + → ('seas') • → ('fluorine') • + → ('male dogs') • → ('
Rwanda') • → ('
Boaz') Spanish features a similar process and even nonsyllabic can occur so that ('right away') is pronounced . It is not often clear, however, whether such sequences involve a semivowel (a consonant) or a diphthong (a vowel), and in many cases, it may not be a meaningful distinction. Although many languages have
central vowels , which lie between back/velar and front/palatal , there are few cases of a corresponding approximant . One is in the Korean diphthong or though it is more frequently analyzed as velar (as in the table above), and
Mapudungun may be another, with three high vowel sounds, , , and three corresponding consonants, , and , and a third one is often described as a non-labialized voiced velar fricative; some texts note a correspondence between this approximant and that is parallel to – and –. An example is (?) ('white'). It has been noted that the expected symbols for the approximant correlates of are or . ==Approximants versus fricatives==