You must set apart ¹  for the Lord your God every firstborn male born to your herds and flocks. You must not work the firstborn of your bulls or shear the firstborn of your flocks.

Suppose a man has two wives, one whom he loves more than the other, ¹  and they both ²  bear him sons, with the firstborn being the child of the less loved wife.

Then ¹  the first son ²  she bears will continue the name of the dead brother, thus preventing his name from being blotted out of Israel.