As stated in the Vedic literature,

tisro varnanupurvyena

dve tathaika yatha-kramam

brahmana-kshatriya-visam

bharyah svah sudra-janmanah

The purport of this verse is that one's first wife must always be sadrisim, or similar to oneself. In other words, an intellectual man should marry an intellectual wife, a heroic man should marry a heroic wife, a business minded man should marry a woman who can encourage him in such activities, and a sudra should marry a less intelligent woman. The wife must be beyond reproach in terms of her background and character and should always be younger than oneself, ideally five to ten years younger. If one desires to marry a second wife, then, as stated in this verse by the word varnanupurvyena and in the verse spoken by Lord Krishna by the word anukramat, one must wait until the first marriage is established and then select a second wife from the next-lower caste. If one marries a third time the wife must be, again, from the next-lower caste. For example, a brahmana's first wife will be a brahmani, his second wife will be from the kshatriya community, his third wife from the vaisya community and fourth wife from the sudra community. A kshatriya may first marry a kshatriya lady and then vaisya and sudra ladies. A vaisya can accept wives from two classes, and a sudra will accept a wife only from the sudra class. By this progression of marriages there will be relative peace in the family. These Vedic marriage injunctions, as mentioned in the previous verse, are especially for those who are not pure devotees of the Lord.