I've seen a number of examples which show an object on a friction-less plane. A vector diagram is used to show that a component of force due to gravity, -Fr, is balanced by the the reaction force (normal force) at 90 deg to the plane, Fr. This leaves Fx, the resultant force parallel to the plane. - see the left hand diagram at the bottom.
What if the size of Fr was drawn in the diagram larger than the size of Fg (see the right hand diagram)? The new resultant force, Fx, is now perpendicular to Fg. It has to be drawn in a vector diagram in the 'shape' of a parallelogram.
Can non-rectangular vector diagrams be used to find the resultant force on the object, Fx?