|Sound change and alternation|
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 Portuguese and Arabic as well as in Ancient Greek, where it was first described.
In some cases, as in the French examples below, crasis involves the grammaticalization of two individual lexical items into one, but in other cases, as in the Greek examples, crasis is the orthographic representation of the encliticization and vowel reduction of one grammatical form with another. The difference between the two is that the Greek examples involve two grammatical words and a single phonological word and the French examples involve a single phonological word and grammatical word.
A coronis (κορωνίς korōnís "curved"; plural κορωνίδες korōnídes) marks the vowel from crasis. In ancient times this was an apostrophe placed after the vowel (i.e., τα᾿μά), but today it is written over the vowel and is identical to the smooth breathing (τἀμά). Unlike a coronis, a smooth breathing never occurs on a vowel in the middle of a word (although it occurs on doubled rho: διάῤῥοια diarrhoea).
The article undergoes crasis with various nouns and adjectives starting in a vowel:
- τὰ ἐμά → τἀμά "my (affairs)"
- τὸ ἐναντίον → τοὐναντίον "on the contrary"
- τὸ αὐτό → ταὐτό "the same"
- τὰ αὐτά → ταὐτά (plural)
καί undergoes crasis with forms of the first-person singular pronoun, producing a long ᾱ (here not written, since it occurs with a coronis):
- καὶ ἐγώ → κἀγώ "and I", "I too"
- καὶ ἐμοί → κἀμοί "and to me"
In modern monotonic orthography, the coronis is not written.
Greek contraction 
Crasis in English usually refers to merging of words, but in Greek it was more general, and referred to most changes called "contraction" in Greek grammar. (But a contraction in which vowels do not change is synaeresis.)
In Greek contraction, two vowels merge to form a long version of one of the two vowels (e + a → ā), a diphthong with a different main vowel (a + ei → āi), or a new vowel intermediate between the originals (a + o → ō).
In general, the accent after contraction copies the accent before contraction. Often this means circumflex accent. But for nouns, accent follows the nominative singular. Sometimes this means a different accent from the uncontracted form — i.e., whenever the ending has a long vowel.
Contraction in Greek occurs throughout the present and imperfect of contracted verbs and in the future of other verbs. There are three categories based on the vowel of contraction: a, e, or o.
Contraction also occurs in nouns, including the contracted second declension.
S-stem nouns undergo contraction with vowel endings.
|-es stem||-os stem|
Some compound nouns show contraction:
- λειτο-εργίᾱ → λειτουργίᾱ "liturgy"
In French, the contractions of determiners are often the results of a vocalisation and a crasis.
For example :
- de le → du, de les → des
- à le → au, à les → aux
- en les → ès
|This section requires expansion. (August 2007)|
In the evolution of the Portuguese language, several words lost some of its vowels in orthographic reforms that were also influenced by a phonological change. Examples:
- door (Old Portuguese) > dor (pain, grief, sorrow, ache, sore, suffering);
- noo (Old Portuguese) > nó (node, knot, tie, joint, hitch);
- seer (Old Portuguese) > ser (to be [as a condition or fate], to exist);
- veer (Old Portuguese) > ver (to look, to eye, to see, to admit);
- rãa (Old Portuguese) > rã (frog, paddock);
In Portuguese, the most frequently observed crasis today 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, or 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"). This contraction turns the clitic a into the stressed word à. Meanwhile, I person going to a bank, to a supermarket or to a market-place would say Vou ao banco, Vou ao supermercado and Vou à feira, respectively.
Crasis also occurs between the preposition a and demonstratives: for instance, when this preposition precedes aquele(s), aquela(s) (meaning "that", "those", in different genders), they contract to àquele(s), àquela(s). In this case, 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 these examples in standard European Portuguese, though this qualitative distinction is not made by most speakers in Brazilian Portuguese (whilst some dialects, as Rio de Janeiro's fluminense, are exceptions).
The crasis is very important and can completely change the meaning of a sentence, for example:
- 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
Further rules 
To determine whether the crasis always apply, or whether one may use the contraction à (with an accent) instead of the preposition a (without an accent), one must pay attention to the rules:
1) it replaces the preposition a by another preposition, as em ("in") or para ("to"). If, with replacement, the definite article a ("the") is still possible, then the crasis applies. Examples:
- Pedro viajou à Região Nordeste: with grave accent, because it equivalent to "Pedro traveled 'to the' Northeast Region". Here, para a Região Nordeste would also be used.
- O autor dedicou o livro a sua esposa: without a grave accent in Brazilian Portuguese, because it is equivalent to "the autor dedicated the book 'to' his wife". A consistent use according to the rules in Brazil would not allow para a sua esposa be used instead. In European Portuguese, nevertheless, rules change, and it is O autor dedicou o livro à sua esposa, albeit it can not be explained translanting to English as both sentences carry the same meaning. In Portuguese-speaking countries other than Brazil, para a sua esposa is to be deemed standard usage.
2) in a supposed situation the nominal complement is exchanged, after "a", from a feminine noun for a masculine noun and if with the exchange, it is necessary to use the combination 'ao' (that is used naturally by native speakers), then the crasis applies. Examples:
- Prestou relevantes serviços à comunidade, he/she paid outstanding services to the community: with a grave accent, because when one changes the object to a masculine noun – "Prestou relevantes serviços ao povo" he/she paid outstanding services to the people - the combination "ao" ("to [masculine] the") appears.
- "Chegarei daqui a uma hora" I will arrive in an hour: without crasis, because when you replace the object to a masculine noun - "Chegarei daqui a um minuto" I will arrive in a minute - the combination "ao" does not appear (as "um/uma", indefinite articles, appear instead of "o/a").
Important: The grave accent is never used before masculine words (nouns, names, 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") when 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.
§ To the exception of
É preciso declarar guerra à guerra! (It is needed to declare war on war!)
É preciso dar mais vida à vida. (One needs to give more life to life)
Optional crasis 
The grave accent is optional in the following cases:
- Before a female's first name
Refiro-me [à/a] Fernanda. ("I am referring to Fernanda")
- Before feminine possessive pronoun
Dirija-se [à/a] sua fazenda. ("Go to your [own] hacienda")
- After the preposition até'
Dirija-se até [à/a] porta. ("Go by that door", "get out of here")
Eu fui até [à/a] Bahia de carro. ("I traveled to Bahia by car")