An experiment found that the brain uses one set of neural circuits to identify the numbers 1–4; these circuits are very specific to their own numbers. A separate set of circuits respond to the numbers 5–9; these are less precise, and are activated by adjacent numbers.
For this reason, it is easier to determine when there are four things than to determine there are five things.
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I don’t think that’s why, not least because this is a recent discovery. The reason is, presumably, because 26+ keys are difficult to fit on a small but still usable keyboard.
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Yes. And they had letters grouped with the numbers because 26+ keys is a lot to fit on a small but still usable keyboard (or dial, originally).