The cases are the following:
197. A dental surd mute or nasal, or the dental sibilant, when immediately preceded by a ṣ, is everywhere converted into the corresponding lingual.
a. Under this rule, the combinations ṣṭ, ṣṭh, and ṣṇ are very common; ṣṣ is rarely so written, the visarga being put instead of the former sibilant (172): thus, jyótiḥṣu instead of jyótiṣṣu.
b. Much less often, dh is changed to ḍh after final ṣ of a root or tense-stem, with loss of the ṣ or its conversion to ḍ: see 226 c.
c. Those cases in which the final ṣ becomes ṭ before su (e.g. dviṭsú: 226 b) do not, of course, fall under this rule.
198. In the other (comparatively infrequent) cases where a dental is preceded by a lingual in internal combination, the dental (except of su loc. pl.) becomes lingual. Thus:
a. A n following immediately a ṇ made such by the rule given at 189, above—or, as it may be expressed, a double as well as a single n—is subject to the lingualization: thus, the participles arṇṇá, kṣuṇṇa, kṣviṇṇa, chṛṇṇá, tṛṇṇá; and, after prefixes (185 a), niṣaṇṇa, pariviṇṇa, viṣaṇṇa, váṣyaṇṇa. But TS. has ádhiṣkanna, and RV. yájuḥ ṣkannám.
b. Only a very few other instances occur: ī́ṭṭe and ā́iṭṭa from √īḍ; ṣaḍḍhā́ (also ṣaḍdhā́ and ṣoḍhā́), and ṣaṇṇā́m (ṣaṣ + nām: anomalous gen. pl. of ṣaṣ: 483). A small number of words follow the same rule in external combination: see below, 199.
c. But tāḍhi (Vedic: √taḍ + dhi) shows loss of the final lingual after assimilation of the dental, and compensatory lengthening.
d. Some of the cases of abnormal occurrence of ḍ are explained in a similar way, as results of a lingualized and afterwards omitted sibilant before d: thus nīḍá from nisda, √pīḍ from pisd, √mṛḍ from mṛsd. For words exhibiting a like change in composition, see below, 199 c.
199. In external combination—
a. A final t is directed to be assimilated to an initial lingual mute: thus, taṭ-ṭīkā, taḍ ḍayate, taṭ-ṭhālinī, taḍ ḍhāukate: but the case never occurs in the older language, and very rarely in the later. For final n before a lingual, see 205 b.
b. An initial dental after a final lingual usually remains unchanged; and su of the loc. pl. follows the same rule: thus, ṣáṭtriṅçat, ā́naḍ diváḥ, ekarā́ṭ tvám; ṣaṭsú, rāṭsú.
c. Exceptions are: a few compounds with ṣaṣ six showing double ṇ (198 b): namely, ṣáṇṇavati, ṣaṇṇābhi (and one or two others not quotable): IB. has ṣaṇ ṇiramimīta.
d. In a few compounds, moreover, there appears a lingualized dental, with compensatory lengthening, after a lost lingual sibilant or its represen-