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A Poset 
 is an interval order if it is Isomorphic to some set of
Intervals on the Real Line ordered by left-to-right precedence.  Formally, 
 is an interval
order provided that one can assign to each 
 an Interval 
 such that 
 in the
Real Numbers Iff 
 in 
.
See also Partially Ordered Set
References
Fishburn, P. C. Interval Orders and Interval Graphs: A Study of Partially Ordered Sets.  New York: Wiley, 1985.
 
Wiener, N.  ``A Contribution to the Theory of Relative Position.''  Proc. Cambridge Philos. Soc. 17, 441-449, 1914.