For rows, the profile between columns can be: (both rows end at the same column), (row ends earlier, row sticks out), or (row ends earlier, row sticks out).
That's states per column. State means both rows are flush. From here, you can place a vertical domino, two horizontal dominoes, or a tromino (which creates an uneven profile). State means row has a protruding cell you must cover. You can place a tromino to fill it and make the profile , or place a horizontal domino in row to bring it flush to .