This is possible, since a boost in, say, the positive z -direction, preserves orthogonality of the coordinate axes. A general boost B (w) can be expressed as L = R (e z, w) B z (w) R (e z, w), where R (e z, w) is a rotation taking the z -axis into the direction of w and B z is a boost in the new z -direction. Each rotation retains the property that the spatial coordinate axes are orthogonal. The boost will stretch the (intermediate) z -axis by a factor γ, while leaving the intermediate x -axis and y -axis in place. The fact that coordinate axes are non-parallel in this construction after two consecutive non-collinear boosts is a precise expression of the phenomenon of Thomas rotation.