Phonological conditioning is the process whereby phonemes become assimilated to match the properties of adjacent phonemes.

The morphs {il-}, {im-}, {in-}, {ir-} at the beginnings of these four words (in which these morphs function as prefixes) are all representatives of a morpheme representing a meaning of negation, thus turning the word to which they become attached into its opposite meaning. In this case, the selection of these prefixed morphs is conditioned by the phonetic properties of the words to which they become attached, or rather, by the phonetic properties of the initial sound of the word to which the prefix becomes attached.

For example: the first sound in the word possible /p/ is a bilabial sound, therefore, the morph {im-} is selected because it ends in a likewise bilabial sound /m/. This so-called phonological conditioning reflects the tendency in language to facilitate (make easier) the pronunciation of sequences of speech sounds. Sounds that are produced in the same area of the vocal tract (by means of the same or nearby articulators) are easier to pronounce in sequence. The principle underlying this observation is called the principle of least (articulatory) effort or linguistic economy. It reflects the tendency of human language (and indeed many other types of human actions) to become maximally efficient by facilitating the articulation of linguistic sound sequences. These variants of a single morpheme are called allomorphs of this morpheme. We can thus say that the morphs {il-}, {im-}, {ir-} have the same meaning and function as the morph {in-}; all of which realize the bound morpheme and prefix [in-]. They can thus be said to be allomorphs of the morpheme [in-].