Crasis (κρᾶσις mingling) is the contraction of a vowel or diphthong at the end of a word with a vowel or diphthong beginning the following word. Over the syllable resulting from contraction is placed a ' called corōnis (κορωνίς hook), as τᾱ̓́λλα from τὰ ἄλλα the other things, the rest.
a. The coronis is not written when the rough breathing stands on the first word: ὁ ἄνθρωπος = ᾱ̔́νθρωπος.
b. Crasis does not occur when the first vowel may be elided. (Some editors write τἄλλα, etc.)