Because of a difference in their distances, two bells appear to be the same size. But larger bells vibrate at lower frequencies than smaller bells. Listening to both, you can deduce that the bell in the tower has a deeper tone -- a lower frequency -- and must be larger in size and therefore farther away. In this analogy the bells are two Cepheid variable stars that have different periods. If the two Cepheids had the same apparent brightness, the one with the longer period (lower frequency) must be farther away.