क्विन्प्रत्ययस्य कुः

The word पदस्य is understood here. The word क्विन्प्रत्यय is a Bahuvrihi meaning 'that stem which has kvin as its affix.' For the final consonant of such a stem, a guttural is substituted. As घृतस्पृक् 3.2.58, हलस्पृक्, मन्त्रस्पृक् ॥

The sutra could have been क्विनः कुः; the word प्रत्यय is used in the sutra to show the Bahuvrihi compound; so that the व् of क्विन् may not be changed to a guttural. Moreover this Bahuvrihi also indicates that the roots which take the क्विन् affix, change their final to a guttural before other affixes than क्विन् ॥ Thus the roots सृज् and दृश् take क्विन् to form स्रक् and दृक् nouns by 3.2.59, 3.2.60. The guttural change will take place even when these roots are declined as verbs: as, मानो अस्राक् मानो अद्राक्, where अस्राक् and अद्राक् are the Aorist of सृज् and दृश् ॥ The augment अट् is not elided, though the मा is added 6.4.75 as a Vedic diversity. The ईट् augment also does not take place as a Vedic irregularity. The augment अम् is added by 6.1.58, and the vowel is lengthened by Vriddhi by 7.2.3. Thus अट् + सृज् + सिच् + तिप् = अस्रज् 6.1.58 = अस्राक् 7.2.3 and 8.2.62. Otherwise it would have been ष by 8.2.36. So also in दृग्भ्यां, दृग्भिः, the श् is changed to a guttural, though the noun दृश् is a क्विप् formed noun and not formed by क्विन्; and this is so, because the verb दृश् does take क्विन् also.

Objection:- If this be so, there ought to be guttural change in रज्जुसृड्भ्यां from the root सृज् with क्विप्? Answer:- The guttural change however in not desired here.

In gutturalisation, ज is changed to ग, and श to ख, which both become क by वावसाने 8.4.56,