It requires some explanation why stress applied to the walls covering the coordinate axes takes a minus sign (e.g. for the left wall we have − − σ σ x x ). For simplicity, let us focus on the left wall with tension − − σ σ x x . The minus sign is due to the fact that a vector normal to this wall n → → = [ − − 1 , 0 , 0 ] = − − e → → x is a negative unit vector. Then, we calculated the stress vector by definition s → → = n → → ⋅ ⋅ σ σ = [ − − σ σ x x , − − σ σ x y , − − σ σ x z ] , thus the X component of this vector is s x = − − σ σ x x (we use similar reasoning for stresses acting on the bottom and back walls, i.e.: − − σ σ y x , − − σ σ z x ).