In a dvandva compound, let a what begins with a vowel and ends with a short a be placed first.,
Thus, उष्ट्रखरम् 'the camel and the ass.' उष्ट्रशशकम् 'the camel and the hare.'
Why do we say 'अत् with a त्' 1.1.70? This rule does not apply when it is long आ as अश्वा + वृषः = अश्वावृषौ or वृषाश्वे .,