Before a taddhita affix having an indicatory ñ , ṇ or k the vṛddhi is substituted for the first vowel of the second member, denoting a mass in its widest sense when the word ardha precedes it, but optionally for the first vowel of ardha,
As अर्धद्रौणिकम् or आर्धद्रौणिकम्, अर्धकौडविकम् or आर्धकौडविकम् with ठञ् 5.1.18. Why do we say when denoting a mass ? Observe आर्धक्रोशिकम् only (= अर्धक्रोशः प्रयोजनमस्य) ॥,