In the chandas (Vedas) at (or ad) is not the substitute of su and am Nominative 1st-Case and Accusative -2nd case endings after itara,
As इतरमितरमण्डमजायत; वार्त्रघ्नमितरम् ॥ Why in the Vedas? See इतरत् काष्ठम्, इतरत् कुड्यम् ॥ Had this sutra been placed immediately after अतोऽम् (VII. I. 24), we could have made it simpler by saying इतराच्छन्दसि; the present position of the sutra indicates that we should divide it into two, to include other cases. In the case of एकतर, the अदड् substitution does not take place, either in the Veda or in the Common Language. As एकतरं तिष्ठति, एकतरं पश्य ॥,