The secondary phonemes are harder to observe than the primary phonemes, because they occur only in combinations or in particular uses of simple forms (e.g. 'John?' in contrast with 'John').
- Bloomfield (1935), a pag.91

[...] secondary phonemes - phonemes which are not part of the simplest linguistic forms, but merely mark combinations or particular uses of such forms.
- Bloomfield (1935), a pag.109

Secondary phonemes, we recall, are phonemes which do not appear in any morpheme, but only in grammatical arrangements of morphemes.
- Bloomfield (1935), a pag.163