A less important point in which our present method of description is inadequate is the phonetic similarity among the allophones of various phonemes. Thus English /p, t, k/ all have identically varying allophones in identically varying positions /strongly aspirated initially, unaspirated after /s/, etc.); /k, g, ŋ/ have identical places of articulation in identical environments (fronted allophones after front vowels, etc.). These similarities are recognized in the grammar when we describe the variation in allophones of all the analogous phonemes in one statement, as was done above. But the similarities among these phonemes are not explicit in the phonemic inventory or directly marked in the transcription.
- Harris (1944), a pag.184

The component can therefore have different phonetic variants (allophones) in various positions, and the environmental factor which determines the particular allophone may be anything outside the component itself: other components with which it is concurrent, neighboring components or pauses, position of the component within the sequence of segments, etc.
- Harris (1944), a pag.187