The vṛddhi is substituted for the first vowels of both (the first and second) members in a compound ending with 1. hṛd 2. bhaga and 3. sindhu before a taddhita affix having an indicatory ñ , ṇ or k ,
As सुहृदयस्येदं = सौहार्दम्, सौभाग्यम्, दौर्भाग्यम् सौभागिनेयः (= सुभगाया अपत्यं), दौर्भागिनेयः 4.1.126. The words सुभगा and दुर्भगा occur in the Kalyanadi class 4.1.126, and the affix ढक् and इन् augment are added. सुभग also occurs in Udgatri class 5.1.129. That word, however, does not get the Vriddhi in the second member, As महते सौभगाय ॥ This is a Vedic anomaly. Similarly from सक्तुसिन्धवः (= सक्तुप्रधानाः सिन्धवः) we have साक्तुसैन्धवः (= सक्तुसिन्धुषु भवः) so also पानसिन्धवः ॥ The word सिन्धु occurs in Kachchhadi class, and सैन्धवः is formed by अण् ॥ The Tadanta-rule applies to words formed there-under.
The words सुहृद् and दुर्हृद् are anomalously formed by 5.4.150. But the reading adopted by Kasika is सुहृदयस्येदम्; so the word hridaya is changed to hrid by 6.3.50 or 6.3.51. The word sindhu means 'a country', 'a river' or 'an ocean'.,