Rule 1:
From top to bottom, the black shape moves two places each time, first down the stairs, until it reaches the bottom. After that, it moves up along the other stairs. On reaching the top of the stairs, the model continues again. This model continues in the next column.
Rule 2:
From top to bottom, the white shape moves one place each time, first down the stairs, until it reaches the bottom. After that, it moves up along the other stairs. On reaching the top of the stairs, the model continues again. This model continues in the next column.
Notice:
When the black shape and white shape coincide, the black shape is shown in the front.