Approximants are speech sounds that involve the articulators approaching each other but not narrowly enough or with enough articulatory precision to create turbulent airflow. Therefore, approximants fall between fricatives, which do produce a turbulent airstream, and vowels, which produce no turbulence. This class of sounds includes lateral approximants like [l] (as in less), non-lateral approximants like [ɹ] (as in rest), and semivowels like [j] and [w] (as in yes and west, respectively).
Before Peter Ladefoged coined the term "approximant" in the 1960s the term "frictionless continuant" referred to non-lateral approximants.
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 corresponds 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, Montreuil (2004:104) remarks that, for example, the final glides of English par and buy differ from French par ('through') and baille ('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 [j] of piede, 'foot', appearing in the nucleus and that of piano, 'slow', appearing in the syllable onset) and Spanish (with a near minimal pair being abyecto, 'abject', vs. abierto 'opened').
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