Literally, a pigeonhole is a small hole in a loft, the nesting-place of a pigeon. This has led to two metaphorical uses:
The term pigeonholing is used to describe processes which attempt to classify disparate entities into a small number of mutually exclusive categories, named for the idea that each pigeon must go into a pigeonhole, but can only be in one hole at a time.
The pigeonhole principle is a mathematical principle based on the observation that when there are more pigeons than pigeonholes, two pigeons must end up in the same hole.
A 'pigeonhole' can also be a small compartment for holding papers.
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The pigeonhole principle is an example of a counting argument which can be applied to many formal problems, including ones involving infinite sets that cannot be put into one-to-one correspondence.
If we assign a pigeonhole for each number of hairs on a head, and assign people to the pigeonhole with their number of hairs on it, there must be at least two people with the same number of hairs on their heads.
For example, if 2 pigeons are randomly assigned to 4 pigeonholes, there is a 25% chance that at least one pigeonhole will hold more than one pigeon; for 5 pigeons and 10 holes, that probability is 69.76%; and for 10 pigeons and 20 holes it is about 93.45%.
The accumulated mail matter is removed from the pigeonholes, and the full complement of supports is shifted to display a selected one of the remaining sets of indicia, each set of which bears a species-genus relationship to one of the indicia of the primary set.
Pigeonholes 14 are formed by a plurality of shelves 16 extending laterally between sidewalls 18 and 18', and a plurality of divider elements 20 extending vertically between the shelves.
Regardless of the number of pigeonholes in the sorting cabinet, the number of destinations which may be accommodated in any one of the pigeonholes associated with the primary set of indicia means and having an alphabetic designation may be no greater than the total number of pigeonholes in the sorting cabinet.
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