The Power Of Double-Doubles

I’ve received several e-mails from viewers basically asking:

“What’s so special about double-doubles? You’ve got two empty spaces left in a box, and you know which numbers go there. They form a double-double. But so what?”

Actually in that case, where there are only two spaces left in a box, you’re right. A double-double is obvious. It may not provide much information at all. But that’s only one situation.

In this article, I’m going to show you a very common situation that will demonstrate how powerful double-doubles are. The power of the double-double in the situation I’m going to show you comes way before there are only two empty spaces left in a box.

If you’ve not already realized the power of double-doubles, when you learn and practice what I’m about to show you, you’ll notice that solving puzzles seem easier, and you’ll be able to solve harder puzzles than before.

Before I get into my little lesson here, I have to emphasize that finding double-doubles relies on one thing: good pencil marking techniques. I hope by now you’ve kicked the brute-force habit, but even if you still have a tendency to put more data on the puzzle that I recommend, that additional data will obscure the good information.

However, if you limit your pencil marks like I show you in all my videos, these double-doubles just pop right out and slap you in the face. You don’t have to search for them.

To demonstrate the power of the double-double, take a look at this Sudoku puzzle. I’ve remove extra boxes and numbers to help us focus on what’s important.


All the action is going to take place in Box 1 (the upper-left 3×3 square). You can easily see, given the other information on the puzzle, that the only places 2 and 4 can go are in the two places indicated with the pencil marks (Row 1 Column 2 and Row 3 Column 2).

This forms what I call a Double-Double: two numbers that can only go in two spaces.

Because the 2 and 4 can only go in those two spaces within the box, we can conclude that nothing else can go in those spaces. If you placed some other number in one of those two spaces, then there would be no place for either the 2 or the 4 in that box. Something would get left out. We’d break the rules.

This is where the power of the double-double comes in. Since nothing else can go there, you can draw other conclusions. In the example above, we can now determine exactly where the 1 goes in that box. It goes in the yellow space.

If you didn’t recognize the 2-4 double-double, and given the limited amount of information available (which is very common on harder puzzles), then the 1 could go in three possible places in box #1. But since we found the double-double, we can exclude two of those three spaces as possibilities.

In this simplified example, this one move of using the double-double to place the 1 leads to several other valuable conclusions. What are those conclusions?

That’s my challenge to you: I want you to try to figure out how many other conclusions you can draw from this simple example.

Now obviously you don’t have a lot of other information, but there are some important and valuable conclusions you can draw, like where certain numbers can go.

I’m not asking you to solve the puzzle. I asking, “Based on what you know, what else can you tell me about where numbers can go?” Hint: You have a think a little outside the box.

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The Professor