Within Teamwork Projects, a task list is automatically marked as complete once each of the individual tasks in the list have been completed.

To mark a task as complete, you can select the tick mark to the left of the task title/assignee.

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Once you have completed all tasks within the list, the list will be marked complete and moved to the Completed Task Lists section on the left pane of the Tasks area.

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For more information, see: Pinning a task list open