Closest Common Ancestor
It's instructive to follow all the black and solid-red lines in an element's Version Browser display. This traces the entire ancestry of real versions of an element. In particular, you can use the real-version ancestry to determine the closest common ancestor of any two versions. This is the most recent version upon which the two versions are both based, by some combination of direct ancestor and merge connections. (When considering a virtual version in a closest-common-ancestor analysis, first follow the green line back to the corresponding real version.)
You can also use the CLI command accurev anc -c to find the closest common ancestor of two versions.
In a Merge operation, AccuRev determines the closest common ancestor of the two versions to be merged, and uses this version to perform a 3-way merge.
See The Merge, Patch, and Reverse Patch Algorithms