A branch collar is the often visible swelling in a woody plant that forms at the base of a branch where it is attached to its parent branch or to the tree's trunk. The top of the branch collar consists of dense interlocking wood grain, which provides mechanical support to the branch attachment. Branch collars can also be flat or somewhat recessed into the trunk or parent branch, as in some conifers. The accretion of layers of wood behind the branch collar produces a conical decay-resistant structure often referred to as the 'branch core'. The knot found in lumber is this branch core.