उरण् रपरः

When a letter of अण् प्रत्यहार comes as a substitute for ऋ it is always followed by र्।
This sutra consists of three words viz., उः which is the genitive singular of ॠ and means literally 'of ॠ' or 'in the place of ॠ'; the second word is अण्, a pratyahara denoting अ, इ and उ long and short; the third word is रपरः which qualifies अण् and means 'having a र after.'

This sutra is useful in fixing the proper substitutes of ॠ. Thus, there are only three guna vowels अ, ए and ओ. Of these what is the proper guna for ॠ? By the application of the rule of 'nearness of place' we see that अ is the nearest substitute. So that अ is the guna of ॠ; and further by this rule, this अ must have a र् after it. Thus though technically speaking अ is the guna of ॠ, the actual substitute is अर्. Thus कृ + ता = कर्त्ता; similarly हर्ता.

So also when ॠ is replaced by इ as by sutra 7.1.100, this substitute इ must be followed by a र्. As कॄ + अति = कि + र् + अति = किरति 'he scatters;' similarly गिरति 'he swallows.'

So also when ॠ is replaced by उ, as by sutra 4.1.115 this उ must be followed by a र्. As द्वि + मातृ + अण् = द्वैमातुरः 'son of two mothers'

This र् is to be added only when ॠ is replaced by अ, इ or उ (अण्) vowels, and not when it is replaced by any other letter.

Thus by 4.1.97, "the ॠ of the word सुधातृ is replaced by अक् (technically अकङ्) when the affix इञ् is added." Thus, सुधातृ + इञ् = सुधात् + अक् + इ = सौधातकिः, 'a descendant of Sudhatri' Here the substitute of ॠ, that is to say, the syllable अक् is not followed by र्.

The र in the text has been taken by some to be a pratyahara formed by the letter र् and अ of लण्; and thus it includes the letters र् and ल्. In that case the sutra would mean that अण् substitutes of ॠ and लृ are always followed by र् and ल् respectively. Thus guna of ri = ar, of li = al.