Simultaneity is the property of two events happening at the same time in a frame of reference. According to Einstein's theory of relativity, simultaneity is not an absolute property between events; what is simultaneous in one frame of reference will not necessarily be simultaneous in another. (See Relativity of simultaneity.) For inertial frames moving at speeds small compared to the speed of light with respect to one another this effect is small and can for practical matters be ignored such that simultaneity can be treated as an absolute property.