There are several words which have vowel variations. This seems like it could
be a fossilization of morphological processes in an older version of Japanese.
How much do we know about these processes? Can we reconstruct a grammatical or
phonological explanation for these vowel variations?

```

    雨: あめ
    雨水: あまみず
    
    月：つき
    月読：つくよみ
    
    高：たか
    丈：たけ
    
```
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2018-07-12T22:21:46.707
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2018-07-12T22:21:46.707
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What do we know about vowel variations such as 雨 read as あめ/あま