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:

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

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:

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.)