When the principal planes or axes of an object are "not" parallel with the projection plane, but are rather tilted to some degree to reveal multiple sides of the object, it is called an axonometric projection. "Axonometric projection" (not to be confused with the closely related "principle of axonometry", as described in Pohlke's theorem) is further subdivided into three groups: "isometric", "dimetric" and "trimetric projection", depending on the exact angle at which the view deviates from the orthogonal. A typical (but not obligatory) characteristic of axonometric pictorials is that one axis of space is usually displayed as vertical.