When we model, we create a number of views of the system being described, usually 2 or 3. For a complete description 3 are normally needed. Each view is an 'orthogonal' view. Each view is needed for a full understanding of the system. Views are orthogonal when:

	Each view has information that is unique to that view
	Each view has information that appears in other views
	The information that is common between views is consistent