H-tree
Beginner
A symmetric branching pattern shaped like nested letter H’s, so every endpoint is the same distance from the center.
Novice
A clock layout that branches in nested H shapes: each wire splits into two equal-length wires, again and again, so every endpoint is the same wire distance from the center.
Expert
Zero skew by symmetry when the loads are uniform, so it is used for the top levels of distribution. Blockages and uneven sink placement break the symmetry, and it uses more wire, and so more power, than a tree shaped around the actual sinks.