The word śvaśura 'father-in-law', is optionally only retained, when spoken of along with śvaśurū 'mother-in-law'.,
Thus श्वशुरः + श्वश्रू may be either श्वशुरौ 'father-in-law and mother in-law,' or श्वश्रश्वशुरौ ॥,
श्वशुरः श्वस्रवा
Adhyāya 1 · Pāda 2 · Rule 71