Compensatory Lengthening Before Historical Post-vocalic /r/ in English

Compensatory lengthening refers to the lengthening of a vowel sound because of the loss of a consonant. The consonant is often in the syllable coda. In non-rhotic varieties of English, compensatory lengthening occurs before the loss of historical post-vocalic /r/.

The following words have compensatory lengthening in non-rhotic varieties of English:

bird
car
fork
four 
girl
north
park
star
world
your

In words with the mid front, high front and high back vowels, many non-rhotic varieties have a schwa following the vowel. The combination of the vowel and schwa can be analyzed as a diphthong. Here are examples:

beer
care
here
there
tour

The schwa also occurs following diphthongs. This is also common in rhotic varieties of English. However, an alternative analysis is that a syllabic /r/ follows the diphthong in rhotic varieties. The schwa occurs in the following words:

choir
fire
hour 
tire
tower

An example of compensatory lengthening in English is long vowels before the loss of historical post-vocalic /r/. Before mid and high front vowels and high back vowels, many non-rhotic varieties also have a schwa. This is also true before diphthongs. Though long vowels occur before the loss of historical post-vocalic /r/, they are not phonemic. They never contrast with short vowels in the same environment.