"Her first husband who sent her away may not marry her again"—Deuteronomy 24:4.
It is forbidden for a man to remarry a woman he divorced if she has remarried [and subsequently been divorced again or widowed] in the interim.
"Her husband's brother shall be intimate with her"—Deuteronomy 25:5.
The brother of a married man who dies childless is commanded to marry his widowed sister-in-law. [See Positive Commandment 217 for the mechanism by which this mitzvah can be bypassed.]
