The branch bark ridge (BBR) is raised bark that forms within the branch crotch. It remains on the trunk as the tree grows. The BBR is the black ridge on the white birch. The BBR is the key to proper pruning.
The BBR shows the angle of the branch core in the tree. On some trees-London plane and smooth-barked eucalypts-the BBR is shed with the outer bark. Still, an indentation remains on the trunk where the BBR formed.
Source Page 26 TREE PRUNING.
Franklin, J. F., H. H. Shugart and M. E. Harmon 1987, Tree Death as an Ecological Process (The causes, consequences, and variability of tree mortality) BioScience Vol. 37 No.8; Pg 550-556