How to make a 3D origami piano

(page 11)

19th picture of 3D origami piano

19.

Rotate two flaps by 90 degrees in the reverse directions.

Prepare to concentrate on the region encircled with a dotted ellipse.

20th picture of 3D origami piano

20.

Let us concentrate on the region encircled with a dotted ellipse according to the step #17.

Prepare to apply a mountain-fold.

Make sure that the folded flap should be the bottom-most layer.