Crasis (in Greek κρᾶσις = combination, mingling) is a phonetic phenomenon which consists in merging two successive words into a single word. Such fusion occurs between words that frequently correlate, when the first word ends in a vowel and the second word starts with a vowel. Crasis originated from the need to ease the pronunciation of consecutive vowels, which Greek tended to prevent in many ways.
The most frequent forms of crasis (some tens altogether) developed from the fusion of the conjunction καί = and or from article forms ending in vowel and followed by common words starting with vowel. Words resulting from crasis are usually characterized by a coronis, a diacritical mark identical to the smooth breathing and placed over the resulting vowel. E.g.: καὶ ἐν = and in > κἀν. Words resulting from crasis of an article with rough breathing (ὁ, ἡ = masc. the, fem. the) carry a rough breathing instead of a coronis (e.g.: ὁ ἄνθρωπος = the man > ἅνθρωπος). When crasis originates an aspirated vowel preceded by a stop consonant, such consonant becomes aspirated (e.g.: τὰ ὅπλα = the arms > θὦπλα). Vowel combination in crasis does not strictly follow contraction rules. The final iota of the first word is dropped (e.g.: καὶ οὐ = and not > κοὐ), whereas iota becomes subscript if it is the second vowel of the initial diphthong of the second word (e.g.: ἐγὼ οἶδα = I know > ἐγᾦδα).
When practicing the language, it will be quite important to identify crasis in words, as well as to trace back the two original merged words, thus allowing proper interpretation. Keep in mind that words resulting from crasis are not always listed as independent headwords in dictionaries.
The following table lists the most frequent forms of crasis together with the original words. This list does not include some extremely rare forms whose occurrences are highly improbable in common usage of the Greek language.
Try to memorize the following chart. Then you can try this self-assessment test on crasis.