linguistic:voice_change, quality:correction
Tenseness

Tenseness is used to describe the opposition of tense vowels vs. lax vowels. This opposition has traditionally been thought to be a result of greater muscular tension, though phonetic experiments have repeatedly failed to show this.

Unlike the other features of vowel quality, tenseness is only applicable to the few languages that have this opposition (mainly Germanic languages, e.g. English), whereas the vowels of the other languages (e.g. Spanish) cannot be described with respect to tenseness in any meaningful way.

One may distinguish the English tense vs. lax vowels roughly, with its spelling. Tense vowels usually occur in words with the final silent ⟨ e ⟩, as in mate. Lax vowels occur in words without the silent ⟨ e ⟩, such as mat. In American English, lax vowels do not appear in stressed open syllables.

In traditional grammar, long vowels vs. short vowels are more commonly used, compared to tense and lax. The two sets of terms are used interchangeably by some because the features are concomitant in some varieties of English. In most Germanic languages, lax vowels can only occur in closed syllables. Therefore, they are also known as checked vowels, whereas the tense vowels are called free vowels since they can occur in any kind of syllable.