Sentence:  A doorknob of whatever roundish shape is effectively a continuum of levers, with the axis of the latching mechanism&mdash;known as the spindle&mdash;being the fulcrum about which the turning takes place. Applying a force tangential to the knob is essentially equivalent to applying one <WSD>perpendicular</WSD> to a radial line defining the lever. 
Meaning of the word marked with <WSD> tags:  at or forming a right angle ("to" something).