Srimad-Bhagavatam: Canto 9 - Chapter 1 - Verse 5

Sanskrit:

श्रीराजोवाचब्रह्मर्षिर्भगवान् काव्य: क्षत्रबन्धुश्च नाहुष: ।राजन्यविप्रयो: कस्माद् विवाह: प्रतिलोमक: ॥ ५ ॥

ITRANS:

śrī-rājovācabrahmarṣir bhagavān kāvyaḥkṣatra-bandhuś ca nāhuṣaḥrājanya-viprayoḥ kasmādvivāhaḥ pratilomakaḥ

Translation:

Mahārāja Parīkṣit said: Śukrācārya was a very powerful brāhmaṇa, and Mahārāja Yayāti was a kṣatriya. Therefore I am curious to know how there occurred this pratiloma marriage between a kṣatriya and a brāhmaṇa.

Purport:

According to the Vedic system, marriages between kṣatriyas and kṣatriyas or between brāhmaṇas and brāhmaṇas are the general custom. If marriages sometimes take place between different classes, these marriages are of two types, namely anuloma and pratiloma. Anuloma, marriage between a brāhmaṇa and the daughter of a kṣatriya, is permissible, but pratiloma, marriage between a kṣatriya and the daughter of a brāhmaṇa, is not generally allowed. Therefore Mahārāja Parīkṣit was curious about how Śukrācārya, a powerful brāhmaṇa, could accept the principle of pratiloma. Mahārāja Parīkṣit was eager to know the cause for this uncommon marriage.