Given a normalized spanning intervals , its complement is a set of normalized spanning intervals whose extension is the complement of the extension of . One can compute as follows:
An important property of normalized spanning intervals is that for any normalized spanning interval , contains at most seven normalized spanning intervals.
The intuition behind the above definition is as follows. First note that the negation of is . Next note that the extension of contains intervals whose endpoints q and r satisfy . Thus the extension of contains intervals whose endpoints satisfy the negation of this, namely . Such a disjunction requires four spanning intervals, the first four in the above definition. Additionally, if the extension of contains intervals of the form [q,r], the extension of will contain all intervals not of the form [q,r], namely (q,r], [q,r), and (q,r). Similarly for the cases where the extension of contains intervals of the form (q,r], [q,r), or (q,r). This accounts for the last three spanning intervals in the above definition.
We now see why it is necessary to allow spanning intervals to have open ranges of endpoint values as well as infinite endpoints. The complement of a spanning interval, such as [[i,j],[k,l]], with closed endpoint ranges and finite endpoints includes spanning intervals, such as , with open endpoint ranges and infinite endpoints.