When I started with Double Disappearing 9 Patch (DD9P) I gave an explanation of what I did but not a real tutorial. I would like to correct that today because I know how nice it is when you're in a bee and you can just link your bee-mates to a tutorial.
To make a double disappearing 9 patch block with this layout you will need:
9 small squares: 5 colors, 4 background
5 big squares: 1 color, 4 background
The size of the squares to cut for a 10" finished block (shown) are:
small squares: 3"
big squares: 4"
Make a nine patch block as shown. Whatever color is in the middle will end up in all four of the final block-units. It doesn't matter where you put the others, they will be cut apart from each other in the next step.
Cut the block down the middle in both directions
Spread the blocks out, keeping their orientation to the center
Use the larger squares to make a new nine patch block
Cut that block down the middle in both directions
Rearrange and sew together
Notes: If you can double the disappearing nine-patch you could triple or even quadruple it! Readers of my blog have done just that. Way to go innovators! You are the reason I love having a blog.
In case you want to start with different sizes for your beginning patches, I figured out the formula for other sizes of DD9P
size of the small blocks = x
size of the large blocks = y
size of finished final block = z
y = 1.5x - 1/2"
z = 3y - 2"
The downside of this block is that all the halving and halving again gives you painful 1/4" and 1/8" cutting problems and weird finished block sizes for pretty much everything except this 10" finished size. That doesn't matter if you just like sewing and cutting and resewing but if you are super into precision or writing patterns for other people you'll get frustrated.
People regularly ask me if I can make the DD9P a 12" finished block and the answer, sadly, is no, I can't. No one can, because the math of cutting a three-unit thing into a two-unit thing won't allow it if you can't cut in thirds and sixths of an inch, and since we don't have tools for that in quilting it's not going to happen. However, if you really really needed a 12" finished block you would start with 3.5" blocks in the first nine patch, and 4.75" blocks in the second nine patch. After you've sewn the final block together it will be 12 3/4" unfinished. You could trim it to 12 1/2" (or just leave it close enough!). I hope that helps.
I love seeing what you make with my tutorials, please link or email if you get a chance!