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The famous 80-20 rule states that, for many events, 80% of the effects come from 20% of the causes. It is a common rule of thumb in business; e.g., "80% of your sales comes from 20% of your clients." From the programmers' perspective, the 80-20 rule means that 80 percent of a program's resources are used by about 20 percent of the code: 80 percent of the runtime is spent in approximately 20 percent of the code; 80 percent of the memory is used by some 20 percent of the code; 80 percent of the disk accesses are performed for about 20 percent of the code; 80 percent of the maintenance effort is devoted to around 20 percent of the code.