Genesis 15:9-10

Take me an heifer... — This form of making a covenant was probably that usual in Babylonia, and thus Abram received the assurance of his inheritance by means of a ceremonial with which he was familiar. But in most ancient languages men are said to cut or strike a covenant, because the most solemn formula involved either the cutting of victims in two, or striking them dead, as was the Roman manner. The severing of the bodies was not, as some suppose, to represent the two parties; but, as explained in Jeremiah 34:18, it set forth the penalty of perjury, and was usually accompanied by the imprecation upon the covenant-breaker of a destruction as complete as that which had befallen the slaughtered animals. There is no mention in this place of a sacrifice, although the animals are those subsequently set apart for sacrifice by the Levitical law. The heifer, she-goat, and ram at three years old would each have attained its full maturity; but there may be a further symbolic meaning in there being three animals each three years old.

Laid each piece... — More exactly, and laid each half over against the other. The birds were not divided; but as there were two, Abram probably placed one on one side and one on the other.