In phonetics, postalveolar is a new name, adopted by the International Phonetic Association in 1993, to include the class of sounds including those formerly known as palato-alveolar. These are the sh ch j sounds of English shah, char, jar, and the zh sound in measure, which doesn't occur initially in native English words.

The term 'postalveolar' is supposed to be a neutral compromise, in that it just describes where the sounds are made: the alveolar ridge is just behind the upper teeth, and English t d n l are alveolar sounds. Postalveolar sounds are made with the tongue a little behind that, towards the hard palate.

However, it seems to me the term creates as many difficulties as it solves, because no less than four different classes of sounds are made in the postalveolar region, and what is important in distinguishing them is the manner of articulation.

  • The ones traditionally described as palato-alveolar are laminal, made with the blade (lamina) of the tongue lying along the region with a groove down the middle for the air to pass through in a diffuse sibilant sound.

  • The English r sound is pronounced in an unusual way: see How to pronounce an English "R" for the dialectal variations. In most British and Southern Hemisphere English the r is an approximant, that is the tongue comes near the postalveolar region but not close enough for friction. This position has always traditionally been called postalveolar.

    English is very unusual in that it has affricates in this position: the groups tr and dr are normally pronounced as a single connected sound. Around the world, Malagasy also has these sounds and Vietnamese has the tr.

    The problem is if the much more common ch j sounds are now called postalveolar too, how are they different from these? The basic difference I think (and believe me, it's very hard for even a trained phonetician to experiment with confidence on their own speech) is that these are apical, that is made with the tip (apex) of the tongue.

    So we would distinguish the two classes of sound as lamino-postalveolar and apico-postalveolar. (The alveolar sounds of English, t d n l, are normally apico-alveolar.)

  • Third, there are the so-called alveolo-palatal or alveo-palatal sibilants that occur in Polish (there written si zi ci) and Chinese (written x q j in pinyin). These are softer than English sh ch j. Real phoneticians (i.e. not just your humble noder) are uncertain how to describe them: they're also laminal, but with a difference: palatalized? As far as I can tell, the tip of the tongue is lowered.

  • Fourth are the class of sounds common in Indian languages, called retroflex. An American English r is also usually retroflex. These don't present a problem, because their characteristic feature is that the tip of the tongue is curled back (retroflexed) to touch or approach the postalveolar region. These are still called retroflex by the IPA, not postalveolar.