First, passengers sitting in the window seats on one side of the plane all board at once, in alternating rows (row 1, 3, 5, etc.). Then the same is done on the other side of the plane. Then the middle seats, still in alternating rows, boards on the first side of the plane. That continues with the other side's middle seats, then (first one and then the other) aisle seats. Then, do it all again for the even-numbered rows.