Clock mesh
Beginner
A grid of clock wires all tied together, so every area of the chip gets the tick from several directions at once.
Novice
A grid of clock wires, all connected together and driven by many buffers at once, with short wires down to the flip-flops. Arrival times come out very even, at a high cost in power.
Expert
The grid puts branch resistances in parallel and the drivers short together, which averages out variation, so skew at the grid is near zero. Costs: wire capacitance, current wasted between drivers that switch at slightly different times, and analysis that needs circuit simulation (SPICE) because the network is not a tree.