An important part of phonology is studying which sounds are distinctive units within a language. For example, the "p" in "pin" is aspirated, but the "p" in "spin" is not. In English these two sounds are used in complementary distribution and are not used to differentiate words so they are considered allophones of the same phoneme. In some other languages like Thai and Quechua, the same difference of aspiration or non-aspiration differentiates words and so the two sounds (or phones) are therefore considered two distinct phonemes.