Look at the image. What does the wrench represent?
I know, I know. It tells you it represents the fulcrum and the perpendicular. But what does that mean, exactly?
The fulcrum is the axis of rotation. The perpendicular is the lever arm.
The perpendicular secures the parallel. This happens through compression.
When you are assessing the perpendicular, it's a body part, like the limb or the torso or the foot.
In the picture, you see the wrench overlaying the body part that is the perpendicular. The part of the wrench that meets the screw is oriented towards a joint.
That's because joints are fulcrums. They are the place where rotation occurs.
The parallel indicates the direction of the rotation. Remember that parallel and perpendicular are complementary pairs; every time you identify a perpendicular, there will be a corresponding parallel.