Genesis 31:51,52

‘And Laban said to Jacob, “See this heap, and see this pillar, which I have set between me and you. This heap be witness, and this pillar be witness, that I will not pass over this heap to you, and that you will not pass over this heap and this pillar to me, for harm.”

Previously the guarding has been against the threat from Laban if his daughters are not rightly dealt with, thus the heap was mentioned. Now the guarding is two way and so both heap and pillar are mentioned. It is significant that Laban fears Jacob. The intervention of God on Jacob's behalf on a dream is not something he can pass over lightly, and he knows how Yahweh has prospered Jacob in the past. Thus he himself wants some warranty that peace will be maintained both ways. The pillar is, as it were, Jacob's signature to the covenant of peace.

“Which I have set.” This does not mean that Laban placed the pillar, only that he sees himself as having caused it to be set as part of the covenant signs. As leader of the confederation, releasing Jacob from it, it is he who sets the terms of the covenant and he sees himself as responsible for all connected with it.