Ou and iu(iou) belong to "seven friends" and can rhyme. Bet twelve Hou in eighteen rhymes, with vowels of ou and iu and rhyme of "Hou".
Rhyme (yā yùn) is also called "rhyme". When writing poems, songs, fu and other rhymes, rhyming words are used at the end of sentences or couplets, which is called rhyme.