An important part of traditional, pre-generative schools of phonology is studying which sounds can be grouped into distinctive units within a language; these units are known as phonemes. For example, in English, the "p" sound in pot is aspirated (pronounced [pʰ]) while that in spot is not aspirated (pronounced [p]). However, English speakers intuitively treat both sounds as variations (allophones) of the same phonological category, that is of the phoneme /p/. (Traditionally, it would be argued that if an aspirated [pʰ] were interchanged with the unaspirated [p] in spot, native speakers of English would still hear the same words; that is, the two sounds are perceived as "the same" /p/.) In some other languages, however, these two sounds are perceived as different, and they are consequently assigned to different phonemes. For example, in Thai, Hindi, and Quechua, there are minimal pairs of words for which aspiration is the only contrasting feature (two words can have different meanings but with the only difference in pronunciation being that one has an aspirated sound where the other has an unaspirated one).