Spanish has six falling diphthongs and eight rising diphthongs. While many diphthongs are historically the result of a recategorization of vowel sequences (hiatus) as diphthongs, there is still lexical contrast between diphthongs and hiatus. There are also some lexical items that vary amongst speakers and dialects between hiatus and diphthong: words like "biólogo" ('biologist') with a potential diphthong in the first syllable and words like "diálogo" with a stressed or pretonic sequence of and a vowel vary between a diphthong and hiatus. hypothesize that this is because vocalic sequences are longer in these positions.