Crasis

Crasis is a type of contraction in which two vowels or diphthongs merge into one new vowel or diphthong, making one word out of two. Crasis occurs in many languages, including French, Italian, Portuguese, and Spanish; 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, in other cases, like in the Greek examples, crasis is the orthographic representation of the encliticization 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.

Greek

In both Ancient and Modern Greek, crasis merges a small word and long word that are closely connected in meaning.
In Ancient Greek, a coronis marks the vowel from crasis. In ancient times, it was an apostrophe placed after the vowel, but it is now written over the vowel and is identical to smooth breathing in Unicode. 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:

τὰ ἐμά → τᾱ̓μά "my "
τὸ ἐναντίον → τοὐναντίον "on the contrary"
τὸ αὐτό → ταὐτό "the same"
τὰ αὐτά → ταὐτά

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

καὶ ἐγώ → κᾱ̓γώ "and I", "I too"
καὶ ἐμοί → κᾱ̓μοί "and to me"

In the modern monotonic orthography, the coronis is not written.

Italian

In Italian, crasis occurs between the prepositions a, da, di, in, con, su, per and the singular masculine definite article il or in fewer cases with the plural masculine definite articles i and gli.

a il → al
da il → dal
di il → del
in il → nel
con il → col
con i → coi
su il → sul
per il → pel
per i → pei
per gli → pegli

French

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

de le → du, de les → des
à le → au, à les → aux
en les → ès

In colloquial Québécois French, crasis extends to form further words.

à + la → à
sur + la → s'a
sur + les → s'es
il + est → yé

Portuguese

The most frequently-observed crasis is now the contraction of the preposition a with the feminine singular definite article a, indicated in writing with a grave accent or the masculine singular definite article o. For example, instead of *Vou a a praia, one says Vou à praia. 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, aquela, which contract to àquele, àquela. The accent marks a secondary stress in Portuguese.
In addition, the crasis à is pronounced lower as than the article or preposition a, as, in the examples in standard European Portuguese, but the qualitative distinction is not made by most speakers in Brazilian Portuguese.
Crasis is very important since it can change the meaning of a sentence:

Exposta, a polícia - The police is exposed
Exposta à polícia - She is exposed to the police
Glória, a rainha - Glória the queen.
Glória à rainha - Glory to the queen
Dê a mulher - Give the woman
Dê à mulher - Give to the woman

These rules determine whether crasis always applies or whether one may use the contraction à instead of the preposition a :
Replace the preposition a by another preposition, as em or para. If after replacement, the definite article a is still possible, crasis applies:

Pedro viajou à Região Nordeste: with a grave accent because it equivalent to "Pedro traveled 'to the' Northeast Region". Here, para a Região Nordeste could also be used.
O autor dedicou o livro a sua esposa: without a grave accent in Brazilian Portuguese because it is equivalent to "The author dedicated the book 'to' his wife". A consistent use, according to the rules in Brazil would not allow para a sua esposa to be used instead. In European Portuguese, the rules are different, and it is O autor dedicou o livro à sua esposa, but in English, both sentences have the same meaning.

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:

Prestou relevantes serviços à comunidade, He/she paid outstanding services to the community: with a grave accent because if the object is changed to a masculine noun, "ao" is now used.
"Chegarei daqui a uma hora" I will arrive in an hour: without crasis because when the feminine noun is changed to a masculine noun, there is no "ao".

The grave accent is never used before masculine words ; verbs; personal pronouns; numerals, plural nouns without the use of the feminine plural definite article as ; city names that do not use a feminine article; the word casa if it has the meaning of one's own home; the word terra when it has the meaning of soil; and indefinite, personal, relative or demonstrative pronouns or aquela); 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 some exceptions:

É preciso declarar guerra à guerra!
É preciso dar mais vida à vida.

Crasis also occurs between the prepositions de, em and por and the definite articles.

de o → do
de a → da
de os → dos
de as → das
em o → no
em a → na
em os → nos
em as → nas
por o → pelo
por a → pela
por os → pelos
por as → pelas

Optional crasis

The grave accent is optional in the following cases:

Before a female's first name
: Refiro-me Fernanda.
Before a feminine possessive pronoun
: Dirija-se sua fazenda.
After the preposition até
: Dirija-se até porta.
: Eu fui até França de carro.

Spanish

In Spanish, crasis occurs between the prepositions a or de and the masculine definite article el.

a el → al
de el → del