(a) Start from the box at the top of the diagram and follow one of the two lines leading out of this box.
(b) Proceed from box to box as long as all successive boxes contain criteria which are applicable to the observed sky.
(c) When this procedure leads to a box with a criterion which is not applicable to the observed sky, go back to this previous box and follow the other line leading out of this box.
(d) If this line leads to a box, repeat the procedure described under (b) and (c). If the line leads to a picture, the code figure below this picture is the correct code to be reported.
(e) If all the successive boxes contain criteria which are applicable to the observed sky, the procedure will finally lead to a box from which two or more lines terminate in pictures. Read the citeria below these pictures to obtain the correct code figure.