The affix cha comes in the chandas (Vedas) in the five-fold senses taught in pantho ṇa nityam [[5.1.76]], tamadhīṣṭo bhṛto bhūto bhāvī [[5.1.80]], after a stem ending in vatsara,
This debars ठञ् ॥ Thus इद्वत्सरीयः, इदावत्सरीयः ॥,
वत्सरान्ताच् छश् छन्दसि
Adhyāya 5 · Pāda 1 · Rule 91