Bhishma said The Brahman, the Kshatriya, and the Vaishya are considered the three twiceborn castes, To marry in these three castes has been ordained to be the duty of the Brahmana, O Yudhishthira.
भीष्म उवाच ब्राह्मणः क्षत्रियो वैश्यस्त्रयो वर्णा द्विजातयः। एतेषु विहितो धर्मो ब्राह्मणस्य युधिष्ठिर॥