• noun:   someone who takes the place of another person
• verb:   exchange people temporarily to fulfill certain jobs and functions
• verb:   do something in turns
• verb:   go back and forth; swing back and forth between two states or conditions
• verb:   be an understudy or alternate for a role
• verb:   reverse (a direction, attitude, or course of action)
• adjective:   of leaves and branches etc; first on one side and then on the other in two ranks along an axis; not paired

  Example: "Stems with alternate leaves"

• adjective:   allowing a choice

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