Ring counter
Ten flip-flops in a loop with exactly one lit — how ENIAC counted in decimal without ever detouring through binary.
A ring counter is a shift register closed into a circle: a single one walks around it, one position per pulse, and the digit is read off as "which flip-flop is lit". ENIAC's twenty accumulators each held ten such decade counters, built from vacuum tubes at two triodes per flip-flop. That accounts for a large share of the machine's 17,468 tubes, and therefore for its 150 kilowatts.
The choice was deliberate rather than naive. The designers wanted intermediate results readable straight off the neon lamps on the panel, and no conversion in either direction when moving data to and from punched cards. The price was the tube count: a binary counter holds a decimal digit in four flip-flops instead of ten. EDVAC switched to binary for exactly that reason, and everything after it followed. The ring counter survives in digital design as the one-hot counter, where designers still trade flip-flops for the absence of decode logic — the same bargain, four technology generations later.