syllables when before , e.g. in the word Valencia 'Valencia' The open allophone is phonetically open-mid , and appears: In open syllables when in contact with , e.g. in the words guerra 'war' and reto challenge In closed syllables when not followed by , e.g. in the word belga 'Belgian' In the diphthong , e.g. in the words peine 'comb' and rey king Mid back vowel The close allophone is phonetically close-mid , and appears in open syllables, e.g. in the word como 'how' The open allophone is phonetically open-mid , and appears: In closed syllables, e.g. in the word con 'with' In both open and closed syllables when in contact with , e.g. in the words corro 'I run', barro 'mud', and roble 'oak' In both open and closed syllables when before , e.g. in the word ojo 'eye' In the diphthong , e.g. in the word hoy 'today' In stressed position when preceded by and followed by either or , e.g. in the word ahora 'now' Open vowel The front allophone appears: Before palatal consonants, e.g. in the word despacho 'office' In the diphthong , e.g. in the word aire 'air' The back allophone appears: Before the back vowels , e.g. in the word flauta 'flute' In closed syllables before , e.g. in the word sal 'salt' In both open and closed syllables when before , e.g. in the word tajada 'chop' The central allophone appears in all other cases, e.g. in the word casa According to Eugenio Martínez Celdrán, however, systematic classification of Spanish allophones is impossible due to the fact that their occurrence varies from speaker to speaker and from region to region. According to him, the exact degree of openness of Spanish vowels depends not so much on the phonetic environment, but rather on various external factors accompanying speech. Diphthongs and triphthongs 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 ('biologist') with a potential diphthong in the first syllable and words like 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. In