hṛd is substituted for hṛdaya before lekha and the affixes yat and aṇ and before lāsa,
Thus हृदयं लिखति = हृल्लेखः, हृदयस्य प्रियं = हृद्यम्, हृदयस्येदम् = हार्दम्, हृदयस्य लासो = हृल्लासः ॥ The word लेख is derived by अण् affix from लिख् ॥ Before the word लेख formed by घञ् affix, this substitution does not take place, as हृदयलेखः ॥ The inclusion of लेख in this sutra proves the existence of this maxim \उत्तरपदाधिकारे प्रत्ययग्रहणे न तदन्तग्रहणम्\ See 6.3.17.,