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.