Double: two puzzles in one board
Every square in a Double puzzle holds two marks, one inside the other, like a donut and its hole. The big squares are one puzzle and the small ones are a second puzzle. They share a grid and nothing else.
How it works
Each layer follows the ordinary rule by itself. In the outer layer, every row, column and box has each mark once. In the inner layer, every row, column and box has each mark once, counted separately. A red outer square and a red inner square can be in the same cell, or in the same row, with no conflict at all.
The two layers do not talk to each other
This is the part people expect to work differently. In many puzzles that stack two grids, the layers give each other clues. Here they do not. The outer answer says nothing about the inner answer. The two puzzles are generated separately, and checked separately.
So the difficulty of a Double is not that the logic is harder. It is that you have to hold two pictures in your head and stop them smearing together. Our eyes are good at seeing a pattern and poor at ignoring one. The challenge is to look at a cell and read only the layer you are working on.
A good way to play it
- Finish one layer, then the other. Most people find it easier to solve the outer squares completely before touching the circles, or the other way round.
- Use the right palette row. The palette has two rows. Tap the outer row to place a big square and the inner row to place a small circle.
- Count each layer on its own. When you ask "what is missing from this row?", ask it twice: once for the squares and once for the circles.
- Do not trust coincidences. If a cell is red outside and red inside, that is a coincidence, not a hint.
Where Double comes from
The idea comes from a quilt of squares inside squares. Look at a quilt like that and you are looking at two layers laid over each other. The quilt page tells the story, and the logo page shows how closely the real fabric behaves like a Double puzzle.
When you will meet it
Double arrives in the levels at Level 61, on a small 4×4 board, and then comes back on bigger boards through the rest of the climb, with Latin Double, turned Double and Double on boards up to 10×10 in the later worlds. In the daily puzzle, Tuesday is always an easy 4×4 Double, and Wednesday, Friday, Saturday and Sunday lean on it as well.