Angle between principal stress and shear stress

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Homework Statement

I know that an element that is subjected to max shear stress is 45 degree away from position of an element that is subjected to principal stress.I couldn't understand why the θ s= θ p -45 degree?
Cant it be θ s= θ p +45 degree ?

Homework Equations

The Attempt at a Solution

IMO, it can be both θ s= θ p -45 degree and θ s= θ p +45 degree [/B]

 

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or i miss out something ?

 

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anybody here ?