You can think of a linear join as a tree with no branches.
Each table in the hierarchy is related to the table above it, until you reach the top table. Tables A, D, and F constitute a linear join hierarchy. Tables C, E, and F form another linear join hierarchy.
In a linear join, each pair of tables represents a one-to-many relationship, in which the lower table of the pair is the “one” side, and the higher table of the pair is the “many” side. Linear join hierarchies can rely on any of the underlying join conditions: key join, natural join, or ON clause join.