मातुरुत् संख्यासम्भद्रपूर्वायाः

The affix अण् comes in the sense of descendant after the word मातृ when it is preceded by a Numeral or by the words सम् or भद्र and the letter उ is substituted for the final ऋ of मातृ।
Thus from द्विमातृ we have द्वैमातुरः "son of two mothers i.e. son of one and adopted by another." द्विमातृ + अण् = द्वैमात् + उ + अ = द्वैमात् + उ + र् + अ (I.I.51.) Similarly षाण्मातुरः, सांमातुरः, and भाद्रमातुरः ॥

This sutra is made in order to teach the substitution of short उ for ऋ of मातृ; for the word मातृ would have taken the affix अण् by the general rule 4.1.83. The sutra contains the word पूर्वायाः in the feminine gender, showing by implication that the rule applies where the word मातृ denotes female. Thus संमिमीते = संमातृ 'he who measures together'. Its patronymic will be सांमात्रः 'son of a fellow-measurer'; because here the word मातृ does not mean 'mother', but a 'measure'. Nor the word धान्यमातृ 'barley measurer' will be affected by this rule. Why do we say "when preceded by a Numeral &c." Observe सौमात्रः 'son of Sumatri'. The word वैमात्रेयः is formed by 4.1.123 as belonging to Subhradi class.