A bottleneck is the stage in a process or system with the lowest capacity, which limits the output of the whole. The term comes from the narrow neck of a bottle, which slows how fast liquid can be poured. In manufacturing, a single slow machine can cap production however fast the other steps run, an idea at the heart of the theory of constraints. In payments and banking, bottlenecks include manual compliance reviews, batch processing windows and correspondent banks that slow cross-border payments. In blockchains, limited block size or throughput creates a bottleneck that shows up as a growing backlog of transactions and higher fees. Removing one bottleneck usually reveals the next one. Also see Bandwidth.