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custom or law decreeing that a widower should, or in rare cases must, marry his deceased wife's sister. The term comes from the Latin word soror, "sister," and was introduced by the British anthropologist Sir James George Frazer. The "sister" may be a biological or adopted sibling of the first wife or a person who is socially classified as such. The sororate often co-occurs with the levirate, or marriage of a widow to her deceased husband's brother. They appear to be the most common of preferential secondary marriages. Either may be permissive rather than obligatory