Scientists have long known that while listening to a sequence of sounds, people often perceive a rhythm, even when the sounds are identical and equally spaced. One regularity that was discovered over 100 years ago is the Iambic-Trochaic Law: when every other sound is loud, we tend to hear groups of two sounds with an initial beat. When every other sound is long, we hear groups of two sounds with a final beat. But why does our rhythm perception work this way?