VOCALIC VERSUS CONSONANTAL
There are a good number of phonetic features. However, two of them are considered to be the most crucial ones: (a) [±consonantal], and (b) [±vocalic]. Their importance is in that they classify all sounds (in the case of English and Farsi at least) into the four major sound classes: (a) consonants, (b) vowels, (c) liquids, and (d) glides:
The distinction between [+consonantal] and [+vocalic] is quite simple. If the air, once out of the glottis, is allowed to pass freely through the oral cavity, the sound is a [+vocalic] phoneme. If the air, once out of the glottis, is partially or totally obstructed in one or more places in the oral cavity, the sound is a [+consonantal] phoneme. It should be noted that the line between [+vocalic] and [+consonantal] phonemes cannot be clearly drawn. A continuum exists between the two extremes. In English, there are also intermediate instances: (a) liquids, and (b) glides. Liquids share features of both [+consonantal] and [+vocalic] sounds. Glides lack features of both [+consonantal] and [+vocalic] sounds.
In order for a phoneme to be [+vocalic], it should meet certain vowel-hood criteria. These criteria include: (a) the degree of openness of the oral cavity also known as the degree of aperture, (b) the degree of tension or laxity of the vocal tract muscles, and (c) amount of duration or length of articulation. There is a large degree of freedom in the articulation of open vowels. However, this freedom is not endless. On the one hand, no vowel can be more open than the standard open vowels (fourth degree of aperture). On the other hand, a vowel could not be much more "close" than the standard close vowels (first degree of aperture). Additionally, the close vowels must have a certain minimum duration in order to be perceived as [+vocalic] rather than [+consonantal].
