SB 9.18.5

śrī-rājovāca
brahmarṣir bhagavān kāvyaḥ
 kṣatra-bandhuś ca nāhuṣaḥ
rājanya-viprayoḥ kasmād
 vivāhaḥ pratilomakaḥ
Word for word: 
śrī-rājā uvāca — Mahārāja Parīkṣit inquired; brahma-ṛṣiḥ — the best of the brāhmaṇas; bhagavān — very powerful; kāvyaḥ — Śukrācārya; kṣatra-bandhuḥ — belonged to the kṣatriya class; ca — also; nāhuṣaḥ — King Yayāti; rājanya-viprayoḥ — of a brāhmaṇa and a kṣatriya; kasmāt — how; vivāhaḥ — a marital relationship; pratilomakaḥ — against the customary regulative principles.
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.