Crasis (;^[1] from the Greek κρᾶσις, "mixing", "blending")^[2] is a type of contraction in which two vowels or diphthongs merge into one new vowel or diphthong, making one word out of two (univerbation). Crasis occurs in Spanish, Portuguese, French and Arabic; it was first described in Ancient Greek.

In some cases, as in the French examples, crasis involves the grammaticalization of two individual lexical items into one. However we in other cases, like in the Greek examples, crasis is the orthographic representation of the enclictization and the vowel reduction of one grammatical form with another. The difference between them is that the Greek examples involve two grammatical words and a single phonological word, but the French examples involve a single phonological word and grammatical word.

In both Ancient and Modern Greek, crasis merges a small word and long word that are closely connected in meaning.^{[n 1]}

In Ancient Greek, a coronis (κορωνίς korōnís "curved"; plural κορωνίδες korōnídes) marks the vowel from crasis. In ancient times, it was an apostrophe placed after the vowel (τα᾽μά), but it is now written over the vowel (τἀμά) and is identical to smooth breathing in Unicode. (For instance, τἀμά uses the character U+1F00 ἀ GREEK SMALL LETTER ALPHA WITH PSILI; psili means smooth breathing.) Unlike a coronis, smooth breathing never occurs on a vowel in the middle of a word although it occurs in a doubled rho: πύῤῥος pyrrhos.

The article undergoes crasis with nouns and adjectives that start with a vowel:

καί undergoes crasis with the first-person singular pronoun and produces a long vowel:

In French, the contractions of determiners are often the results of a vocalisation and a crasis:

The most frequently-observed crasis is now the contraction of the preposition a ("to" or "at") with the feminine singular definite article a ("the"), indicated in writing with a grave accent or the masculine singular definite article o (also "the"). For example, instead of *Vou a a praia ("I go to the beach"), one says Vou à praia ("I go to-the beach"). The contraction turns the clitic a into the stressed word à. Meanwhile, a person going to a bank, a supermarket or a marketplace would say respectively Vou ao banco, Vou ao supermercado or Vou à feira.

Crasis also occurs between the preposition a and demonstrative such as when the preposition precedes aquele(s), aquela(s) (meaning "that", "those", in different genders), which contract to àquele(s), àquela(s). The accent marks a secondary stress in Portuguese.

In addition, the crasis à is pronounced lower as /a/ than the article or preposition a, as /ɐ/, in the examples in standard European Portuguese, but the qualitative distinction is not made by most speakers in Brazilian Portuguese (some dialects, like Rio de Janeiro's fluminense, are exceptions and make the distinction).

Crasis is very important since it can change the meaning of a sentence:

These rules determine whether crasis always applies or whether one may use the contraction à (with an accent) instead of the preposition a (without an accent):

Replace the preposition a by another preposition, as em ("in") or para ("to"). If after replacement, the definite article a ("the") is still possible, crasis applies:

If the nominal complement is changed after "a" from a feminine noun to a masculine noun, and it is now necessary to use 'ao' as used naturally by native speakers, crasis applies:

The grave accent is never used before masculine words (nouns, pronouns, etc.); verbs; personal pronouns; numerals, plural nouns without the use of the feminine plural definite article as ("the"); city names that do not use a feminine article; the word casa ("house") if it has the meaning of one's own home; the word terra ("earth") when it has the meaning of soil; and indefinite, personal, relative or demonstrative pronouns (except the third person and aquele(s) or aquela(s)); between identical nouns such as dia a dia "day by day", "everyday", "daily life", gota a gota "dropwise", "drip", and cara a cara "face to face"; and after prepositions. Here are exceptions:

É preciso declarar guerra à guerra! (It is necessary to declare war on war!)

É preciso dar mais vida à vida. (It is necessary to give more life to life.)