Human speech has two distinctive yet complementary functions and modes. The Overt mode is spoken forwards and is primarily under conscious control. The Covert mode is spoken backward and is not under conscious control. The backward mode of speech occurs simultaneously with the forward mode and is a reversal of the forward speech sounds.
These two modes of speech, forward and backward, are dependent upon each other and form an integral part of human communication. One mode cannot be fully understood without the other mode. In the dynamics of interpersonal communication, both modes of speech combined communicate the total psyche of the person, conscious as well as unconscious.
Covert speech develops before overt speech. Children speak backwards before they do forwards. Then, as forward speech commences, the two modes of speech gradually combine into one, forming an overall bi-level communication process.