Chapter I THE FUNCTIONAL ASPECT OF SPEECH SOUNDS
This chapter is concerned with the linguistic function of speech sounds, i. e. “segments of speech”. We are going to discuss here the definitions of the phoneme, methods used in establishing the phonemic structure of a language, the system of English phonemes, modifications of sounds in connected speech.

1.1. The Phoneme

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1.1. The Phoneme
1.1.1. The definition of the phoneme
To know how sounds are produced by speech organs it is not enough to describe and classify them as language units. When we talk about the sounds of a language, the term “sound” can be interpreted in two rather different ways. In the first place, we can say that [t] and [d] are two different sounds in English, [t] being fortis and [d] being lenis1 and we can illustrate this by showing how they contrast with each other to make a difference of meaning in a large number of pairs, such as tie – die, seat – seed, etc. But on the other hand if we listen carefully to the [t] in let us and compare it with the [t] in let them we can hear that the two sounds are also not the same, the [t] of let us is alveolar, while the [t] of let them is dental. In both examples the sounds differ in one articulatory feature only; in the second case the difference between the sounds has functionally no significance. It is perfectly clear that the sense of “sound” in these two cases is different. To avoid this ambiguity, the linguist uses two separate terms: “phoneme” is used to mean “sound” in its contrastive sense, and “allophone” is used for sounds which are variants of a phoneme: they usually occur in different positions in the word (i. e. in different environments) and hence cannot contrast with each other, nor be used to make meaningful distinctions.
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1.1. The Phoneme
1.1.1. The definition of the phoneme 1.1.2. The phoneme as a unity of three aspects