Chunking theory assumes that chess players encode most of their long-term memory knowledge as chunks – perceptual units that can be treated as wholes. Chunking theory explains the skill effect in recalling chess positions by assuming that strong players are more likely to recognise chunks on a board position, since they have stored many more chunks in long-term memory. In work with Kevin Gilmartin, Herbert Simon used computer simulations and mathematical extrapolations to estimate that a chess master must have learnt about 50,000 chunks. Simon and William Chase's genius was not only to have proposed a powerful theory, but also to have supported it empirically with elegant experiments. In comparison to the original study, another important addition was a copy task, which provided critical information about the structure of chunks. The chunking hypothesis has also been directly confirmed in several experiments. Unfortunately, several experimental findings turned out to be inconsistent with chunking theory.