Constraints on the permissible combination of sounds in a language.
This type of exercise with minimal sets also allows us to see that there are definite patterns in the types of sound combinations permitted in a language.
The first minimal set in Table from Minimal Pair and Set does not include forms such as lig or vig. According to dictionary, these are not English words, but they could be viewed as possible English words.
Phonological knowledge of the pattern of sounds in English words would allow us to treat these forms as acceptable if, at some future time, they came into use.
Example, begin as invented abbreviations (I think Bubba is one very ignorant guy. ~ Yeah, he’s
a big vig!). Until then, they represent “accidental” gaps in the vocabulary of English.