Euphony

Euphony means "beautiful sounds". It refers to words and word-sounds (made of up "phonemes" - pho- = sound)

Beautiful sounds

J. R. R. Tolkien used the term "phonaesthetic" to describe words and sounds that join together to make pleasant sounds, which is now generally known as "euphony." Tolkien popularized, for example, "cellar door", words which naturally go together.

Assimilation

Euphony is related to "assimilation," which is the process by which letter sounds (phonemes, which are the most basic sounds of words) are "harmonized" for easier and more "euphonic" articulation of letter and word combinations.

The process is guided by the "principle of least articulatory effort" or "linguistic economy" (see phonological conditioning [linguisticsweb.org]).

It is the reason why we say "a cow" and "an owl."

With prefixes, different sounds of prefixes can mean the same thing, but they are used in order to "harmonize" the sounds of the prefix with the first letter or sound of the root. From allomorph [linguisticsweb.org], these prefixes all mean the same thing, "not" or negation:

• inadequate
• illogical
• impossible
• irreplaceable

What guides the use of im- versus il-, ir-, and in- are, per that website:

• im- is selected when the word to which the prefix becomes attached starts with a bilabial sound
• il- is selected when the word to which the prefix becomes attached starts with the phoneme l
• ir- is selected when the word to which the prefix becomes attached starts with the phoneme r
• in- is selected in any other phonetic environment, i.e. before any other consonant or before vowels