| 
 | 
 | 
The difference 
, where 
 and 
 are Hinges.  It is the same as the Interquartile Range
for 
, 9, 13, ... points.
See also Hinge, Interquartile Range, Step
References
Tukey, J. W.  Explanatory Data Analysis.   Reading, MA: Addison-Wesley, p. 44, 1977.