159. With the exceptions above stated, the collision of surd and sonant sounds is avoided in combinations — and, regularly and usually, by assimilating the final to the following initial, or by regressive assimilation.
Thus, in internal combination: átsi, átti, atthás, attá (√ad + si etc.); çagdhí, çagdhvám (√çak + dhi etc.); — in external combination, ábhūd ayám, jyóg jīva, ṣáḍ açītáyaḥ, triṣṭúb ápi, dig-gaja, ṣaḍ-ahá, arcád-dhūma, bṛhád-bhānu, ab-já.
160. If, however, a final sonant aspirate of a root is followed by त् t or थ् th of an ending, the assimilation is in the other direction, or progressive: the combination is made sonant, and the aspiration of the final (lost according to 153, above) is transferred to the initial of the ending.
Thus, gh with t or th becomes gdh; dh with the same becomes ddh, as buddhá (√budh + ta), runddhás (√rundh + thas or tas); bh with the same becomes bdh, as labdhá (√labh + ta), labdhvā́ (√labh + tvā).
a. Moreover, h, as representing original gh, is treated in the same manner: thus, dugdhá, dógdhum from duh — and compare rūḍhá and līḍhá from ruh and lih, etc., 222 b.
b. In this combination, as the sonant aspiration is not lost but transferred, the restoration of the initial aspiration (155) does not take place.
c. In dadh from √dhā (155 e), the more normal method is followed; the dh is made surd, and the initial aspirated: thus, dhatthas, dhattas. And RV. has dhaktam instead of dagdham from √dagh; and TA. has inttām instead of inddhām from √idh.
161. Before a nasal in external combination, a final mute may be simply made sonant, or it may be still further assimilated, being changed to the nasal of its own class.
Thus, either tád námas or tán námas, vā́g me or vā́n̄ me, báḍ mahā́n or báṇ mahā́n, triṣṭúb nūnám or triṣṭúm nūnám.
a. In practice, the conversion into a nasal is almost invariably made in the manuscripts, as, indeed, it is by the Prātiçākhyas required and not permitted merely. Even by the general grammarians it is required in the compound ṣáṇṇavati, and before mātrā, and the suffix maya (1225): thus, vān̄máya, mṛnmáya.
b. Even in internal combination, the same assimilation is made in some of the derivatives noted at 111 d, and in the na-participles (957 d). And a few sporadic instances are met with even in verb-inflection: thus,
