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hartley

A unit of information, the amount of information inherent in the choice of one of 10 equally possible states (compare bit). It is equal to log₂10 bits, about 3.321928095 bits.

To see why this value makes sense: 1 bit can designate 1 of 2 possible states (0,1), 2 bits one of 4 possible states (00, 01, 10, 11), 3 bits 1 of 8 possible states (000, 001, 010, 011, 100, 101, 110, 111), and 4 bits 1 of 16 possible states. So the amount of information in designating 1 of 10 choices obviously must fall between 3 and 4 bits.

It is named for Ralph Vinton Lyon Hartley (1888 – 1970).