Because workweeks can be defined as any fixed, continuous 168-hour period, employees working a 9/80 schedule are not automatically eligible for overtime.
A 9/80 schedule has one eight-hour day in its two-week cycle, so an employee's “workweek” can start and end in the middle of their eight-hour shift day so as to not exceed 40 hours of work in that set period. To curtail overtime costs on a 9/80 schedule, employees should only be allowed to trade shifts that do not cause them to exceed 40 working hours in that 168-hour window.