For purposes of counting syllables and assigning stress in Spanish, where an unmarked high vowel is followed by another vowel the sequence is treated as a rising diphthong, counted as a single syllable—unlike Portuguese and Catalan, which tend to treat such a sequence as two syllables. A syllable is of the form "XAXX", where "X" represents a consonant, permissible consonant cluster, or no sound at all, and "A" represents a vowel, diphthong, or triphthong. A diphthong is any sequence of an unstressed high vowel ( or ) with another vowel (as in or ), and a triphthong is any combination of three vowels beginning and ending with unstressed high vowels (as in or ). Hence Spanish writes (no accent), while Portuguese and Catalan both put an accent mark on "" (all three languages stress the first ). The letter is not considered an interruption between vowels (so that is considered to have two syllables: "ahu-mar"; this may vary in some regions, where is used as a hiatus or diphthong-broking mark for unstressed vowels, so the pronunciation would be then "a-hu-mar", though that trait is gradually disappearing).