If all possible combinations of one, two, or three consonants were permitted to begin an English word, there would be more than 12,000 different combinations. The actual number is fewer than 60. The other 11,040 possibilities are prohibited by the rules of English phonology. Phonology has two levels. Phonetics is the physical and concrete level and the psychological or mental aspect is the abstract level.
Speakers consistently recognize and interpret physically different sounds as the same sound.
It is practically impossible to pronounce words, even the same word, in exactly the same way twice, an incredible variety of speech sounds reaches our ears every day. The size and shape of the vocal tract varies from one speaker to the next, and since speech sounds derive from anatomical functions, different anatomies produce different sounds. The knowledge of a sound and its variant are part of the speakers' phonological competence . A mental unit of speech is a phoneme . Phonemes are contrastive sound units which make a difference in words.