A penultimate i or u is lengthened, when the r or v of a root can stand at the end of a word.,
Thus गीः, धूः, पूः, आशीः ॥ These are all examples of roots ending in र् ॥ Of roots ending in व्, examples will be given in the next sutra.
Why have we used the word 'penultimate'? Observe अबिभर् भवान्, here the इ of the reduplicative syllable is not to be lengthened.
Why do we say 'of इक् vowels'? So that the अ of भ in the above example अबिभर् may not be lengthened.
The word धातोः 'of a root' is understood here also. Therefore इ and उ are not lengthened in अग्निः, वायुः ॥
The word पदस्य is understood here also, therefore, not here, गिरौ, गिरः ॥,