a simplified procedure to determine whether two sounds represent the same or different phonemes. the cases on the extreme left and the extreme right are those in which the sounds are allophones.
in phonology, an allophone (/; from the greek ἄλλος, állos, "other" and φωνή, phōnē, "voice, sound") is one of a set of multiple possible spoken sounds, or phones, or signs used to pronounce a single phoneme in a particular language. for example, in english, [t] (as in stop [stɒp]) and the aspirated form [tʰ] (as in top [ˈtʰɒp]) are allophones for the phoneme /t/, while these two are considered to be different phonemes in some languages such as thai and hindi. on the other hand, in spanish, [d] (as in dolor [doˈloɾ]) and [ð] (as in nada [ˈnaða]) are allophones for the phoneme /d/, while these two are considered to be different phonemes in english.
the specific allophone selected in a given situation is often predictable from the phonetic context, with such allophones being called positional variants, but some allophones occur in free variation. replacing a sound by another allophone of the same phoneme usually does not change the meaning of a word, but the result may sound non-native or even unintelligible.
native speakers of a given language perceive one phoneme in the language as a single distinctive sound and are "both unaware of and even shocked by" the allophone variations that are used to pronounce single phonemes.