The substitute n replaces the final i of pati before the feminine affix ṅīp when the word so formed means 'a wife' who takes part in the sacrifices of her husband.,
Thus पत्नि वाचं यच्छ ॥ The feminine पत्नी means legitimate wife, she who is entitled to take part in the sacrifices of her husband. The word यज्ञसंयोग is an Instrumental Tatpurusha compound and means \related through sacrifice\ i. e who shares the fruits of and is entitled to participate in, the sacrifices of her husband. But when the sense is not that of 'wife', the feminine of पति will be पति as ग्रामस्य पतिरियं ब्राह्मणी 'this Brahmani is the head or mistress of this village'. If the word पत्नी means 'the wife who takes part in the sacrifices of her husband', it cannot be applied to the wife of a Sudra, for a Sudra cannot perform a sacrifice, a fortiori not his wife? The term पत्नी is, however, extended to the wife of a Sudra also, by analogy, as the marriage of a Sudra also takes place by offering sacrifice to Fire and making it a witness.,