द्वि-त्रिभ्यां तयस्य अयज् वा

As द्वाववयवावस्य = द्वयम् or द्वितयम्, त्रयम् or त्रितयम् 'a couple' 'a triad' 6.4.148.

The word तय is used in the sutra to indicate the sthanin or thing to be replaced. Had it not been used, the sutra would have run thus द्वित्रिभ्यामयज् वा \ayach optionally comes after dvi and tri\. Here ayach would have become a separate affix, instead of being a mere substitute. Where is the harm in its being a separate affix? Then the word त्रयम् or द्वयम् will not form their feminine in ङीप्; but अय being taken as sthanivat to तय (1. 1. 56), the affix ङीप् which by 4.1.15, applies to तय, applies to अय also. Thus we have त्रयी गतिः ॥ Moreover by 1.1.33, words ending in तय are declined as Pronouns optionally before nominative plural termination. By the rule of sthanivat adesa 1.1.56, the words ending in अय will also be Pronouns. As द्वये or द्वयाः त्रये or त्रयाः ॥ The च् in अयच् is for accent, showing that the substitute 'aya' differs in accent from the sthani taya.,