we do not need a complete truth verification procedure  but only a procedure for constraint processing  Bennett could reduce the result  proving that also RCC    is NP complete These results  even if they are in general negative for practical purposes  encouraged researches in the direction of restricted models in such a way that at least for certain cases we may process a finite set of RCC constraints in polynomial fashion on deterministic machines  In particular  Nebel sshowed that reasoning with the basic relations of and RCC   are tractable problems  Renz and Nebel RCC
improved the results above  by showing that there exists a maximal tractable subclass of RCC
denoted by H  formed by relations out of which includes all the basic relations  and a maximal to tractable subclass of RCC    denoted by H 8  formed by relations out of including the basic relations A maximal tractable subclass A is a subset of a constraint algebra such that problems defined on A are tractable  while problems defined on proper supersets of A
are not  These results have been obtained in a similar fashion to the work of Nebel and on temporal reasoning   The result of Renz and Nebel is anyway incomplete Burckert since he simply proved that there exists one maximal tractable subclass  and he did not Jonsson and Drakengren to classify every maximal tractable subclasses of RCC showed that there exist four maximal tractable subclasses of RCC    one including the basic relations The result is obtained in a similar fashion to  Drakengren     Jonsson Jonsson Drakengren A complete analysis of the RCC 
Backstrom maximal tractable subclasses including the basic relations has been provided by Renz in Our result is the analogous in MC of Jonsson and Drakengren result for RCC
The MC
algebra  we describe in this paper  is structured in the same way as the Algebra of Partially Ordered Time  PO time algebra   studied by Anger  Mitra and Rodriguez We would like to stress two main aspects here
- The MC algebra has the PO time algebra are the same algebraic structure the discussion of this paper stands a different interpretation of the PO time algebra  even different from the spatial interpretation provided by Anger  Mitra and Rodriguez in

their papers
- The computational results we present here can be applied to the PO time algebra as well  and they extend the previous findings obtained by Anger  Mitra and Rodriguez In particular Anger  Mitra and Rodriguez proved that path consistency is insufficient to ensure consistency for relations in this algebra and that there exists a tractable subalgebra of it which can be treated by an O n 3 algorithm showed that deciding the consistency of a PO time Anger  Mitra and Rodriguez nnetwork is an NP complete problem by reducing to it the analogous decision problem on RCC
Some important observations are needed  with respect to the results of Anger  Mitra and Rodriguez
- The results on the complexity of PO time algebra can be applied to MC
only if we can show that when a MC network is consistent then we can exhibit a consis tent scenario in which vertices of the network are substituted with spatial regions Therefore  even if  we can show this correspondence for PO time algebra too  our NP completeness result is independent  Moreover  the method we used to prove the