The energy-time uncertainty relation is \(\Delta E \cdot \Delta t \geq \frac{\hbar}{2}\). Energy \(E\) and time \(t\) are complementary observables that cannot both be known precisely simultaneously. This is analogous to the position-momentum uncertainty: \(\Delta x \cdot \Delta p_x \geq \frac{\hbar}{2}\). So 'E' is paired with 't' in the uncertainty principle.