[ابو بيشو]

During the flight she sat on the-chair behind me.
During the flight she sat in the seat behind me.
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seat = a place to sit, as found in a cinema, train, bus etc: 'To be on the
safe side, you'd better reserve a seat.'
chair = a movable seat for one person: 'Before the children go home,
they have to put all the chairs on top of the desks.