Crasis

Crasis is the contraction of a vowel or diphthong at the end of a word with a vowel or diphthong at the start of the following word. It occurs, for example, in Portuguese, Arabic, and Greek.

Portuguese
In Portuguese, the most frequent crasis is the contraction of the preposition a ("to" or "at") with the feminine singular definite article a ("the"), indicated in writing with a grave accent. For example, instead of *Vou a a praia ("I am going to the beach"), one says Vou à praia ("I am going to-the beach"). This contraction turns the clitic a into the stressed word à.

Crasis also occurs between the preposition a and demonstratives: for instance, when this preposition precedes aquele, aquela (meaning "that one", with different genders) or aqueles, aquelas (plural), they contract to àquele, àquela, àqueles, àquelas. In this case, the accent marks a secondary stress.

In addition, the vowel à is pronounced lower than the vowel a in these examples in standard European Portuguese, though this qualitative distinction is generally not made in Brazilian Portuguese.