Alternate

every second one of a series

serving or used in place of another

occurring by turns; first one and then the other

of leaves and branches etc; first on one side and then on the other in two ranks along an axis; not paired

someone who takes the place of another person

be an understudy or alternate for a role

reverse (a direction, attitude, or course of action)

do something in turns

go back and forth; swing back and forth between two states or conditions

exchange people temporarily to fulfill certain jobs and functions