तिष्य-पुनर्वस्वोर् नक्षत्र-द्वन्द्वे बहुवचनस्य द्विवचनं नित्यम्

When there is a collective compounding of the words Tishya and Punarvasu (meaning two groups of stars), the final compound is always in the dual. Thus उदितौ तिष्यपुनर्वसूदृश्येते 'the Tishya and Punarvasu having risen are seen (dual).'

There is one star called Tishya; and there are two stars called Punarvasu. As the Dvandva compound of these stars refers to more than two stars, it ought to have been in the plural number. This rule teaches that there should be a dual number, where otherwise there would have been a plural number. Why do we say 'Tishya' and 'Punarvasu'? Observe विशाखानुराधाः 'the stars Visakha and Anuradha.' The compound is in plural number.

This rule would not apply if these words do not connote stars but are used in any other sense. As तिष्यपुनर्वसवो माणवकाः, 'Boys called Tishya and Punarvasus.'

Why is the word 'star' repeated in the sutra, when the previous sutras and the context show that stars were being treated of. The repetition is for the sake of including the synonyms of Tishya and Punarvasu. As तिष्यपुनर्वसू; पुष्यपुनर्वसू, सिद्धपुनर्वसू ॥

The rule only applies when these words form a Dvandva compound, and not when they are compounded in any other way. Thus तिष्यपुनर्वसवः is an example of Bahuvrihi compound meaning \persons who confound the Tishya with the Punarvasu.\

The rule only applies to the plural Dvandva. If the Dvandva takes the singular termination, this rule has no application. As तिष्यपुनर्वसू इदमिति.

This sutra indicates by implication (jnapaka) that \every Dvandva compound may optionally take the terminations of the singular\ for otherwise the employment of the term बहुवचनस्य in the sutra would be superfluous.

The word 'always' has been used in the text to show that the governing power of \option\ stops here with this sutra and does not extend to it or any further.,