alternate

someone who takes the place of another person

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

exchange people temporarily to fulfill certain jobs and functions

be an understudy or alternate for a role

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

do something in turns; "We take turns on the night shift"

every second one of a series; "the cleaning lady comes on alternate Wednesdays"; "jam every other day"- the White Queen

serving or used in place of another; "an alternative plan"

occurring by turns; first one and then the other; "alternating feelings of love and hate"

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