In historical phonology, prothesis refers to the addition of a sound at the beginning of a word without changing its morphological structure, as in Spanish esfera from Latin sphaera (without prothesis the word would have become *sfera).

Before the Divine Liturgy began, the priest's meticulous prothesis set the tone for the solemn Eucharistic celebration.

In his paper, the linguist argued that h-prothesis explains the unexpected onset in certain dialects.

In some dialects of Scottish Gaelic, t-prothesis occurs before vowel-initial words to ease pronunciation.