The vṛddhi is the single substitute of a or ā of a preceding word and the initial diphthong of the succeeding (a or ā + diphthong = vṛddhi),
The word आत् is understood here. For the diphthong which follows an अ or आ, and for the अ or आ which precedes a diphthong, in the room of both these preceding and succeeding आ or आ and diphthong, there is a single substitute viz., the Vriddhi. This debars guna taught in the last sutra. Thus ब्रह्म + एडका = ब्रह्मैडका, खट्वा + एडका = खट्वैडका, ब्रह्म + ऐतिकायनः = ब्रह्मैतिकायनः, खट्वा + एतिकायनः = खट्वैतिकायनः ॥ ब्रह्म + ओदनः = ब्रह्मौदनः, खट्वा + ओदनः = खट्वौदनः, ब्रह्म + औपगवः = ब्रह्मापगवः, खट्वा + औपगवः = खट्वौपगवः ॥,