Transistor stack
Beginner
Several switches in a row that must all be on for electricity to get through.
Novice
Transistors connected in series, source to drain, as in the pull-down network of a NAND. The current has to pass through every one, so a stack conducts less well than a single transistor of the same size.
Expert
Series devices: resistance adds, internal nodes add diffusion capacitance, and upper devices suffer body effect because their sources sit above the rail. Stacks are upsized by their depth for equal drive, and they cut off-state leakage roughly tenfold for a two-high stack.