short vowels, only short vowels or only long vowels. Additionally, vowels that ordinarily functioned as the first and second elements of diphthongs were called ('prefixable') and ('suffixable'). The category of included both diphthongs and the spurious diphthongs , which were pronounced as long vowels in the Classical period. The categories and roughly correspond to the modern terms continuant and stop. Greek grammarians placed the letters in the category of stops, not of continuants, indicating that they represented stops in Ancient Greek, rather than fricatives, as in Modern Greek. Stops were divided into three categories using the adjectives ('thick'), ('thin'), and ('middle'), as shown in the table below. The first two terms indicate a binary opposition typical of Greek thought: they referred to stops with and without aspiration. The voiced stops did not fit in either category and so they were called "middle". The concepts of voice and voicelessness (presence or absence of vibration of the vocal folds) were unknown to the Greeks and were not developed in the Western grammatical tradition until the 19th century, when the Sanskrit grammatical tradition began to be studied by Westerners. The glottal fricative was originally called ('breath'), and it was classified as a , the category to which the acute, grave, and circumflex accents also belong. Later, a diacritic for the sound was created, and it was called pleonastically ('rough breathing'). Finally, a diacritic representing the absence of was created, and it was called ('smooth breathing'). The diacritics were also called and ('thick accent' and 'thin accent'), from which come the Modern Greek nouns and . Reconstruction The above information is based on a large body of evidence which was discussed extensively by linguists and philologists of the 19th and 20th centuries. The following section provides a short summary of the kinds of evidence and arguments that have been used in this debate, and gives some hints as to the sources of uncertainty that still prevails with respect to some details. Internal evidence Evidence from spelling Whenever a new set of written symbols, such as an alphabet, is created for a language, the written symbols typically correspond to the spoken sounds, and the spelling or orthography is therefore phonemic or transparent: It is easy to pronounce a word by seeing how it is spelled, and conversely to spell a word by knowing how it is pronounced. Until the pronunciation of the language changes, spelling