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छन्दांसि नाम द्विपदां वरिष्ठ स्वच्छन्दयोगेन भवन्ति तत्र ।
In plain words
As for the metres — O best of men, there they arise of their own accord, by their own free will.
Close to the Sanskrit
The metres indeed, O best of the two-footed, come to be there through the operation of their own will.