The basic idea of Arithmetic Coding is: a message is encoded by an interval of real numbers from the unit interval . The output of Arithmetic Coding is a binary representation of the boundaries of the corresponding interval. This binary representation is incrementally generated during message processing. Starting with the unit interval, for each observed character the interval is made smaller, essentially in proportion to the probability of the character. A message with low information content (and high corresponding probability) is encoded by a comparatively large interval, whose precise boundaries can be specified with comparatively few bits. A message with a lot of information content (and low corresponding probability) is encoded by a comparatively small interval, whose boundaries require comparatively many bits to be specified.
Although the basic idea is elegant and simple, additional technical considerations are necessary to make Arithmetic Coding practicable. See  for details.