ठस्येकः

For ठ at the beginning of a तद्धित affix there is substituted इक्।
As आक्षिकः, शालाकिकः (ठक् 4.4.1), लावणिकः (ठञ् 4.4.52). In the affixes ठक्, ठञ् &c, if the affix is the consonant ठ्, and अ is only for euphony, then here also the अ is for pronunciation only: but if on the contrary, the aggregate ठ (ठ् + अ) is the affix, then the same is the case here. This rule does not apply in Unadi affixes always, as कणेष् ठः = कण्ठः (Unadi l. 103), for there is diversity (बहुल) in the Unadi.

माथितिकः (= मथितं पण्यमस्य 4.4.51) is thus formed मथित + ठक् = मथित् + इक 6.4.148 theअ is dropped)). Now arises the doubt, should इक be replaced by क as taught in the next aphorism 7.3.51, as it comes after a त् ॥ This substitution, however, does not take place, because it was इक् which caused the elision of अ, and now त् which became final by such elision cannot cause the destruction of इक its producer (सन्निपातलक्षणो विधिरनिमित्तं तद्विघातस्य). Or the elided अ 6.4.148 may be considered as sthanivat, and would thus prevent क substitution.

There are two views about this ठ ॥ Some say that the consonant ठ् only is the sthani, and the अ is only for the sake of pronunciation. The others hold that ठ, the consonant and the vowel are sthani in the aggregate. In the first view, the rule would apply to the consonant ठ् at the end of roots like पठ् in पठिता, पठितुं; in the other view the rule would apply to the affix अठच् in कर्मठः ॥ Hence, the commentary uses the word, that ठ must be the cause of अङ्ग ॥