The <backup> element
The <backup> and <forward> elements are required to coordinate multiple voices in one part, including music on multiple staves. The <backup> element is generally used to move between voices and staves. Thus it does not include <voice> or <staff> elements. Duration values should always be positive, and should not cross measure boundaries or mid-measure changes in the <divisions> value.
In this order
This element is used in the following examples: