For a 12" block you'll need:
(6) Half-Square Triangle units, light/dark, 4" finished
(so, you'll start with 4-7/8" light and dark squares cut in half once diagonally,
then sewn together in pairs)
(3) 4-1/2" squares dark
Arrange as shown:
I admit that I don't have a quilt with this block, but I'm thinking that I need to make one. I can see it in scraps (but then, you might have noticed that I can see a LOT of quilt quilts in scraps!), and since the leaf colors are so absolutely gorgeous here this week, I'm seeing the scraps in autumn colors.
O, this would look lovely as a traditional red-and-white, or blue-and-white quilt.
MK: Using the Slideshow feature of the new BlockFab-iPad app (available any day now), I saw this interesting combination of the Double X block with the Rail Fence block.