The substitutes eta and ita come instead of idam when a prāgdiśīya affix beginning with ra and tha respectively follows.,
This debars इश् ॥ The अ in र is for the sake of euphony. Thus इदम् + र्हिल = एतर्हि 5.3.16; इदम् + थमु = इत्थम् 5.3.24.,
एत-इतौ र-थोः
Adhyāya 5 · Pāda 3 · Rule 4