There is a particular group of phonetic features which has customarily been separated from the rest of the linguistic material even though simultaneous with it. This is pitch and stress. The extraction of these features out of the flow of speech is due to the fact that they constitute morphemes by themselves, independent of the rest of the speech, with which they are simultaneous. In 'You. :You? : Yes. : Yes?' We have four different sound-sequences, and four different meanings. These must therefore have four different phonemic compositions. This requirement would be satisfied if we had phonemic /U/ and /E/ as high-pitched vowels contrasting with low-pitched /u/ and /e/. Then we would write /yuw/, /yuw/, /yes/, /YES/. However, the pitch features which are symbolised by /U, E/ have the specific meaning of interrogation. We therefore wish to consider some part of /yuw/, /YES/ as the morphemes ‘you’, ‘yes’ and another part as the morpheme ‘interrogation’. This can be done only if we consider /U, E/ to consist of two simultaneous components /u, e/ and /′/. Then the phonemes /u, e/ are part of the morphemes for ‘you’ and ‘yes’; and the phoneme /′/, or rather the rising pitch which extends over the whole utterance, is the morpheme for interrogation.
- Harris (1944), a pag.182

In most languages that have been investigated, pitch and stress have been found to constitute the elements of special morphemes (such as phrase and sentence intonation or the English contrastive stress). These elements are pronounced simultaneously with the other morphemes of the language. It would be impossible to isolate the other morphemes without extracting the pitch and stress morphemes that occur simultaneously with them. Perhaps as a result of this, it has been customary to extract pitch and stress features even when they form part of the phonemic make-up of ordinary segmental morphemes (words and parts of words). Thus we do not usually say that a language has ten vowels, five loud and five weak, but rather that it has five vowel phonemes plus two degree of stress.
- Harris (1944), a pag.182