It's between United and American airlines.
United is hoping to take over six more gates, claiming the Chicago-based airline operated more than 52% of total flight departures last year, but only had 48% of the gates.
American Airlines has filed a lawsuit against the city to prevent that.
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United sent a letter to employees Monday calling the lawsuit a last-ditch effort.
The airline said American has occupied 41% of O'Hare's gates, while only operating 37% of departures from the airport.
A final decision about the gates is expected in June.