[[File:Maxwell-Distr.png|thumb|300px|In some cases, [[statistical physics]] uses ''probability measures'', but not all [[measure theory|measures]] it uses are probability measures.<ref name=stern>''A course in mathematics for students of physics, Volume 2'' by Paul Bamberg, Shlomo Sternberg 1991 ISBN 0521406501 [http://books.google.com/books?id=eSmC4qQ0SCAC&pg=PA802 page 802]</ref><ref name= gut>''The concept of probability in statistical physics'' by Yair M. Guttmann 1999 ISBN 0521621283 [http://books.google.com/books?id=Q1AUhivGmyUC&pg=PA149 page 149]</ref>]]
Intuitively, the additivity property says that the probability assigned to the union of two disjoint events by the measure should be the sum of the probabilities of the events, e.g. the value assigned to "Heads or Tails" in a coin toss should be the sum of the values assigned to Heads and Tails.
*''Probability and Measure'' by Patrick Billingsley, 1995 John Wiley ISBN 9780471007104
*''Probability & Measure Theory'' by Robert B. Ash, Catherine A. Doléans-Dade 1999 Academic Press ISBN 0120652021