He is serving in the military as a sergeant.

A back vowel is a sound where the back of the tongue approaches the palate.

Back vowels are vowels that are pronounced using the back of the tongue.

A central vowel is a vowel that is pronounced in the central part of the tongue.

The central vowels in Japanese are 'a', 'e', 'o'.

Front vowels are pronounced using the front part of the tongue.

A front vowel is a vowel that is articulated with the front part of the tongue.

The Great Vowel Shift played a significant role in the history of English.

He is writing a thesis on the octovocalic theory.

A diphthong refers to the phenomenon where two vowels are pronounced consecutively within a single syllable.