Illustration of reference frame for a stationary observer in the Schwarzschild case, defined as in Eqs. (27)–(28). The observer (black dot) carries a left-handed system of axes, f₁ f₂ f₃, with f₁ pointing radially towards the central black hole, f₂ pointing tangentially to r = const. Two spheres are opposite to the vector e_ϕ of the local coordinate basis, and f₃ orthonormal is to both f₁ and f₂ (opposite to the vector e_θ of the local coordinate basis).