The compound called avyayībhāva is also avyaya or indeclinable.,
The Avyayibhava or adverbial or indeclinable compounds are formed by joining an indeclinable particle with another word. The resulting compound, in which the indeclinable particle forms generally the first element, is again indeclinable, and generally ends, like adverbs, in the ordinary terminations of the nominative or accusative neuter. Thus अधिहरि 'upon Hari,' अनुरूपं 'after the form i.,e., accordingly.'
The word च in the sutra shows that here ends the definition of avyaya. The avyayas, therefore, are all those words which are comprised in the above five sutras.
What is the object gained by making the avyayibhava compounds avyayas? Three objects are gained, by making these compounds, indeclinables, viz., we can apply to them (1) the rule relating to the elision (लुक्) of the case affixes and feminine affixes, i.e., sutra 2.4.82 thus, in the following example, the words upagni and pratyagni though qualifying the word salavah, do not take the gender and number of the latter, उपाग्नि प्रत्यग्नि शलाभाः पतन्ति (2) the rule relating to accent when an avyaya is compounded with the word mukha, (मुख स्वर), i.e., sutras 6.2.167 & 6.2.168. Thus, उपाग्नि मु॑खः, here, sutra 6.2.167, required the final vowel to take the udatta accent, but the preceding word being an avyaya, sutra 6.2.168, intervened, and the first word retains its natural accent: (3) the rule relating to the change of visarga : into स्, this change being technically called उपचारः thus उपपयःकारः, उपपयःकामः, as compared with अयस्कारः Here, the compound उपपयः being treated as avyaya sutra 8.3.46, applies and prevents the change of: into स्.
The four sutras 1.1.38, 1.1.39, 1.1.40, and 1.1.41 have all been enunciated under the svaradigana. see sutra 1.1.37. These sutras properly, therefore, belong to the ganapatha; their repetition here in the Ashtadhyayi indicates that these rules are अनित्य or not of universal application, viz., all rules relating to avyayas in general, do not apply to them. Thus 2.3.69. declaring that the genitive case is not used along with an avyaya is not a rule of universal application, for we see indeclinables like उदेतोः governed by the genitive case as, सूर्यस्योदेतोः; or क्रूरस्य विसृपः.,