Theory of phonetics

II. Functions of phonemes.
The opposition of phonemes serves to distinguish the meaning of the whole phrase as well. For example, He was heard badly and He was hurt badly.
Thus we may say that the phoneme can fulfill the distinctive function. Secondly, it is material, real and objective. It is realized in speech of all English-speaking people in the form of speech sounds, that is the allophones belonging to the same phoneme are not identical in their articulatory content through there remains some phonetic similarity between them. The allophones which do not undergo any distinguishable changes in the chain of speech (dark, done, dot, etc.) are called principal. There are quite predictable changes in the articulation of allophones that occur under the influence of the neighbouring sounds (God thanks, riddle, etc) in different phonetic situations. Such phonemes are called subsidiary. The examples below will show the articulatory modifications of the phoneme [d] in various phonetic contexts:

• 
  [d] is slightly palatalized before front vowels, deal, did;
  
• 
  [d] is pronounced without any plosion before another stop, bedtime, bad dog, good pat;
  
• 
  [d] is pronounced with the nasal plosion before nasal sonorants [n] and [m], garden, admire, could not, etc.;
  
• 
  The plosion is lateral before the lateral sonorant [l], middle, rapidly, good luck, etc.;
  
• 
  [d] followed by [r] becomes post-alveolar, dry, drive, etc.;
  
• 
  when [d] is followed by the labial [w] it becomes labialized, dweller, etc.;
  
• 
  [d] followed by the interdental [T] and [D] it becomes dental, bad things,
  

read the text, etc.;

• 
  in the initial position [d] is partially devoiced, doctor, deep, etc.;
  
• 
  in the intervocalic position or when followed by a sonorant [d] is fully
  

voiced, leader, drip;

• 
  in the word-final position it is voiceless, road, caused, etc.
  

Thus, allophones of the same phoneme never occur in similar phonetic contexts, they are quite predictable according to the phonetic environment, but they cannot differentiate meanings.