t is substituted for the final of the aṅga (stem) ap before a case-ending beginning with bh,
As अद्भ्याम्, अद्भिः, अद्भ्यः ॥ Why beginning with a भ? Observe अप्सु ॥
In the Veda, त् is substituted before भ, for the final of स्ववस्, from सु + अव् + असुन्; meaning सोभनमवोयेषां; स्वतवस् from the root तु meaning स्वं तवो येषां; मास and उषस्, as स्ववद्भिः, स्वतवद्भिः, माद्भिरिष्टा इन्द्रो वृत्रहा, समुषद्भिरजायथाः ॥ The word मास becomes मास् by (VI. I. 63).,