Traditionally, Lindy Hop was danced to the swing music of the big band era, and this musical style has left its mark in the form of the dance. Swing music is normally in 4/4 time (or sometimes 2/2 time), which means that there are four beats in each bar of music. However, individual musical phrases are often larger than a bar—typically 2 or 4 bars long in swing music, corresponding to 8 or 16 beats.
Obviously, the individual notes in a piece of music vary in length—some notes are held for exactly one beat, some are held for longer and some are held for less time. A common feature of swing music is that when a series of shorter notes are played, the first of each consecutive pair of notes is held for longer than the second of the pair—the two note lengths are in a ratio of 3:2 or 2:1, rather than straight 1:1. In written music this is marked by the phrase "with a swing", and the notes are all written as eighth notes even though they are of differing lengths.
In modern circles, Lindy Hop is danced to a much wider variety of music. This is always in 2/2 or 4/4 time, with tempos varying from around 120 to around 260 beats per minute.
The Musical Interpretation chapter later in the book includes more information on swing music and its interaction with Lindy Hop choreography.