तत्र तेन+इदम् इति सरूपे

The word तत्र means 'a word in the locative case,' and तेन 'a word in the instrumental case.' The word सरूप or 'similar form' applies to both. The word इति indicates that the meaning of the compound so formed should be learned from popular usage, therefore it means, 'seizing, striking, fighting.' All these senses are implied by इति. The word is exhibited in the locative case, if the sense is that of seizing; and the word is exhibited in the instrumental case, when the sense is that of striking; the word indicated by the word इदमिति is युद्ध; as केरोषुकेशोषु च गृहीत्वा इदं युद्ध वृत्तं = केशाकेशि 6.3.137 and (V.4. 127.) ' hair to hair, fighting by pulling each other's hair'; कचाकचि 'hair against hair'; दण्डैश्च दण्डैश्च प्रहृत्य इदं युद्धं वृत्तं = दण्दादण्डि 'stick against stick, fight with stick and stave;' so also मूशलामूशली. In the above examples the samasanta affix is added at the end by rule 5.4.127; and all such words are Aryaya or indeclinable. The final vowel of the first term is lengthened by Rule 6.3.137.

Why do we say 'having the same form'? We cannot form such a compound from the following : हलैश्च मुषलैश्च प्रहृत्य इदं युद्धं वृत्तम् .,