Introducing English Linguistics

Place of articulation assimilation with negative prefixes 
The negative prefixes im-,  in-,  il-, and ir- all mean ‘not’ in words such as
impertinent or illegal. Exactly which of these three prefixes is used depends
upon the place of articulation of the consonant beginning the base to
which they are affixed. If the base begins with a bilabial sound, then [im]
will be used:
immodest [immɑdist]
imperfect [impfεkt]
immobile [immoυbəl]
implausible [implɔzəbəl]
If, on the other hand, the stem begins with an alveolar or velar consonant,
then [in] or [iŋ] will be used:
intangible [intænd əbəl]
incomplete [iŋk mplit]
inauspicious [inɔspiʃəs]
indefensible [indifεnsəbəl]
If the stem begins with [l] or [ɹ], the prefixes [il] and [iɹ] are used, respec-
tively:
illegal [illigəl]
illicit [illisit]
irrelevant [iɹɹεləvənt]
irredeemable [iɹɹədiməbəl]