Let us consider the English phoneme [d]. It is occlusive, forelingual, apical, alveolar, lenis consonant. This is how it sounds in isolation or in such words as door, darn, down, etc, when it retains its typical articulatory characteristics. In this case the consonant [d] is called principal allophone. The allophones which do not undergo any distinguishable changes in speech are called principal.
Allophones that occur under influence of the neighboring sounds in different phonetic situations are called subsidiary, e.g.:
a. deal, did - it is slightly palatalized before front vowels
b. bad pain, bedtime - it is pronounced without any plosion
с. sudden, admit - it is pronounced with nasal plosion before [n], [m]
d. dry - it becomes post-alveolar followed by [r].
If we consider the production of the allophones of the phoneme above we will find out that they possess three articulatory features in common - all of them are forelingual lenis stops. Consequently, though allophones of the same phoneme possess similar articulatory features they may frequently show considerable phonetic differences.
Native speakers do not observe the difference between the allophones of the same phoneme. At the same time they realize that allophones of each phoneme possess a bundle of distinctive features that makes this phoneme functionally different from all other phonemes of the language. This functionally relevant bundle is called the invariant of the phoneme. All the allophones of the phoneme [d] instance, are occlusive, forelingual, lenis. If occlusive articulation is changed for constrictive one [d] will be replaced by [z]: e. g. breed - breeze, deal — zeal, the articulatory features which form the invariant of the phoneme are called distinctive or relevant.
To extract relevant features of the phoneme we have to oppose it to some other phoneme in the phonetic context.
If the opposed sounds differ in one articulatory feature and this difference brings about changes in the meaning this feature is called relevant: for example, port — court, [p] and [k] are consonants, occlusive, fortis; the only difference being that [p] is labial and [t] is lingual.
The articulatory features which do not serve to distinguish meaning are called non-distinctive, irrelevant or redundant. For example, it is impossible to oppose an aspirated [ph] to a non-aspirated one in the same phonetic context to distinguish meaning.
We know that anyone who studies a foreign language makes mistakes in the articulation of sounds. L.V. Shcherba classifies the pronunciation errors as phonological and phonetic. If an allophone is replaced by an allophone of a different phoneme the mistake is called phonological. If an allophone of the phoneme is replaced by another allophone of the same phoneme the mistake is called phonetic.