What is the reason for changing "in-" to "im-" and "con-" to "com-"?

The transformation of "in-" to "im-" and "con-" to "com-" primarily serves to enhance euphony, which refers to the quality of being pleasing to the ear. This change occurs when the prefixes are attached to roots that begin with certain consonant sounds. For instance, "im-" is used before words starting with "b," "m," or "p" (as in "impossible," "immortal," and "imperfect") in order to create a smoother transition between the prefix and the root. Similarly, "com-" works better with roots that start with "b" (like "combine") or "p" (like "compose").

This alteration helps maintain the flow and phonetic harmony of the words, thereby making them easier to pronounce and more pleasant to articulate. The practice of modifying prefixes for phonetic reasons is common in the English language, contributing to the overall aesthetic of spoken language.