6 It shall be, that the firstborn whom she bears shall succeed in the name of his brother who is dead, that his name not be blotted out of Israel.

Matthew Henry's Commentary on Deuteronomy 25:6

Commentary on Deuteronomy 25:5-12

(Read Deuteronomy 25:5-12)

The custom here regulated seems to have been in the Jewish law in order to keep inheritances distinct; now it is unlawful.