# The Power Of Double-Doubles

“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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### What Other’s Say

The videos are a great help and are time saving. [Puzzles] are a lot easier to solve when you don't have too much information. — Clewood

Hi Chad: Your videos are terrific and I have sent your website info to a few friends and hopefully they will contact you and get started. Have a fine day. Cheers, — J. Rutherford

Hello there Professor, I love your lessons because your teaching makes it so simple for me. I watch my husband faithfully work [Sudoku puzzles], but he has no patience with my slow understanding, just like I won't let him teach me golf, being that I hit left handed. Keep sending them. — Laura Calhoun

Hey Chad, Thanx for your commitment. I think you have given me a great heads up on the matter. I never knew how addictive it could be. I think I'm going to have to take up Sudoku anonymous therapy classes now. So long and thanx again. — S. Williamson

Hi Chad: I enjoy your lessons very much. We get two newspapers and they each have a puzzle, so that keeps me busy most of the time. I also have a magazine of puzzles. Being an 86 year old, it gives me a great excuse to sit and WORK. HAVE A GOOD WEEK. — H. Miller

I find your instructions to be very helpful, as a matter of fact after looking at the 3rd lesson, I was able to figure out how to complete the puzzle. Because of your instructions I have bought a Sudoku puzzle book and I am finding the easy and medium puzzles a snap. — Troy

Hi Chad-I'm just a beginner and your lessons are very important. I look forward to the next one. — Pete S.

Chad: I watched all four lessons & enjoyed them. I am able to do the easy puzzles so far with little difficulty and I had never done them before. I would have to say that the lessons have been very helpful. Thanks so much. — J. Rivas

Hi Chad, Your lessons are FANTASTIC!!! Thank you so much...I am enjoying them very much and I have forwarded them to friends as well. Your "character" and your method is GREAT! My girl friend and I (both retired engineers) just love it. Thanks so much. Sincerely, — Bruce

Professor Sudoku, Your Lessons are clear, concise, helpful and talked me off the ledge... I'm determined to "get this" and with your help I'm sure I'll succeed. Thanks! — Cynthia

Your videos are excellent! I'm looking forward to your advanced videos for us that have mastered the more basic puzzles. Thank you. — Gino C.

Hey Sudoku Professor, Thank you very much for you help. I'm up to the Junior 1 & 2 Puzzles, and today I finally made it through a "four star" newspaper puzzle that took me days to finish... but I did it!! The "too much information" rule is, I think, the most helpful rule I've learned. I'm looking forward to the Senior puzzles and think maybe I'll be able to finish my 4 stars sooner after I work through those. I'm addicted! Sincerely, — Kathy

Chad, I think you videos are excellent. I am still learning, and I sometimes replay them all to refresh my memory. I am a senior, and I find I cannot retain things like I did when younger - like you, I have told several of my friends about your lessons, and told them how to get the videos. I am sure you will be hearing from some of them. Thank you again...I look forward to the next lesson. — Fred

The videos helped with the notation and logic. Thanks. — B. Irvine

Hey Prof. The lessons are appreciated. They are a more organized approach than I have used before. So they are a help. Thanks. — Jack Rhine

THANK YOU!!! Where do I begin, I wanted to send you this information but, I didn't want to seem like I was bragging. I have done all the puzzles, including the ones you just gave information on how to look at the puzzle from different ways using your system. IT WORKS..... I'VE LEARNED SO MUCH FROM YOUR LESSONS I APPRECIATE YOU BEING THERE .. Thank You Again. — Philip

We have really enjoyed these...my son and I...he was having difficulties doing Sudoku at school. Looking forward to the next. — Sue, Australia

I have just started doing sudoku puzzles and was pretty much lost till I stumbled onto your website. Thank you so much for your videos. They are wonderful, and I have found success after watching your videos. You are the best thing I have found to help me understand how to solve a sudoku puzzle. I don't know what to say except thank you so much and keep up the good work. I may have given up the puzzles if I hadn't found your web site. Thank You so much. — Lonnie Lloyd (Hermiston, Oregon)

Hello Professor, When I bought your video's I was pretty new at Sudoku and thought the only way to solve them was to use the "Brute Force Technique". After watching some of your video's I can see where the numbers go more easier than before. The techniques that you teach are very easy and straight forward it was worth the money . I would also like to say that the outfit that you wear in the video's is a nice touch. — C. Oake

Hey Professor... As a beginning Sudoku player I searched the web, found your site and subscribed. At the time I was brute-forcing my games with real poor results. After watching your video's on 1-9, doubling, and searching for patterns, I am now completing the Sudoku puzzles in the local weekly paper. I'm looking forward to receiving more lessons and maybe moving my game up the rung of difficulty a notch. Thanks again. — "Tumbleweed"

I am very impressed with how easy you make learning Suduko. Having never played this game prior to receiving your first lesson, I am feeling like I am getting it. Each lesson has helped me even more. I will highly recommend your services to anyone who asks. — Jim Warner, Texas

Thank you for the lessons this is all new to me you have made it easy. I feel I have it. — Rose

Hey, your lessons and method of teaching are great! I just finished winning a game in record time using these lessons. Your technique of going through the numbers was helpful, and your concept of "too much information" was particularly helpful. I am looking forward to the next lesson. — F. Tinker

Good Morning Professor, Just finish watching your First lesson and already know more about solving these Sudoku Puzzles than I did for the last few years. Can't wait to see what is next Thank you, — Charles

Dear Sudoku Professor: Thank you for emailing me the lessons, I am now anxiously waiting to receive the 5th lesson and then I wonder what is next. I love watching your videos, and I have them all saved. I have learned a lot and from your techniques... I am a very busy housewife and a grandma I hardly get free time to solve much. I have also forwarded your website to some of my friends. Again thanks a lot for the videos. — Guity

Due to your instructions, I have been playing Sudoku and deeply enjoying the game. I have not passed up a game yet thanks to you. Most profound thanks. — Ernest M.

So far so good, you definitely make yourself understood. An idiot could understand. Please keep up the good work. Thanks again! — Kathy Connell

I do enjoy the videos. I find them helpful. I received a book of 90 "easy" Sudoku puzzles and find that I am able to solve most of them. I did look at a couple of other online sudoku aids but as you note, I found them difficult to work with. I generally try one [puzzle] each day, and as I said I do figure out most of them. Thanks for your help, — Sheldon

My 6 1/2 year old grandson is enjoying your videos. He is learning Sudoku and is doing quite well. Jacob and I watch your lessons together and we talk about what we see. Thank you. — Ann

Great stuff, Thanks for the lessons. I am using your two and three in box or column primarily, and when that fails, stepping back and filling all the spaces in the mostly filled areas. I owe you. Thanks, — Gary Cramer

Thanks for the lessons. I am usually fair at the easy ones but at my age, 65, I need to push my brain to get some gray matter exercise, and you help with that!!!!!!!!!!!!!!!! — J. Kimball

Your videos were very helpful in learning how to play Sudoku. Thank you very much for creating them and sharing them with us. — Susan Allen

Mr. Chad Barker, I recently bought your Sudoku lessons. I was doing Sudoku close to 1 year, I read several courses that were free on the Internet, and I could not grasp when they explained, like you mentioned, x-wing, xyz-wing, Swordfish, etc, etc. After using your method after 1 or 2 days I realized I could complete diabolical Sudoku's in about 30 to 35 minutes. Now I only works on diabolical Sudoku and have fun doing it. Best regards, — Michel

I viewed all four of the videos you emailed me and they were great. Thank you — Carole

Greetings, Sudoku Prof (SP for short) Pursuant to your request for feedback, may I say without reservation that your video lessons are clear and to the point, which is just what they should be. Using your techniques are improving my game considerably. Thanks! Your humble student, — Arlene

I have just started doing sudoku puzzles and was pretty much lost till I stumbled onto your website. Thank you so much for your videos. They are wonderful, and I have found success after watching your videos. You are the best thing I have found to help me understand how to solve a sudoku puzzle. I don't know what to say except thank you so much and keep up the good work. I may have given up the puzzles if I hadn't found your web site. Thank You so much. — Lonnie Lloyd, Hermiston, Oregon

Hi Chad. I started to play sudoku about 2 months ago and never used to mark any other numbers in until I got to the hard puzzles. And that's when it got to be too much information just like you said. That is when I contacted you for help. Thanks and looking forward to more lessons. — Mike

Thanks, Chad, this is great! Yes, I'll be happy to spread the word. — Connie

Hi, Chad I truly enjoy your Sudoku lessons. Explanations are clear and detailed (just great !). Keep on your good work, Chad : I just can't wait for my next Sudoku lesson ! Have a nice day and take care. — R. GERVAIS

Love it!!!!! I've definitely enjoyed the teaching videos, and they have been a great help to me. I have a better time doing them. Thank you. — Suzanne D.

Hi, Chad, I just got lucky and found your lessons right after I'd started doing sudoku. I've been a crossword person for years and finally decided I had to see what all the hoo-ha was about. The lessons are great. Thanks again, — Tamara

Hi Chad I am impressed with your videos. I found them interesting and fun. I can now do the easy level of sudoku, and some of the intermediate level puzzles. Best Regards, — Louise

Thank you very much, Professor. I did enjoy the videos, they are very helpful and easy to understand. I'm sure this appreciation is shared by many of your students. Wishing you all the best. — Agnes

I thought my method for solving was good, but your method is much better and results in a quicker solution . . . . — P. Hochstetler

Great job. All the other sites want to sell you something before you learn anything. I take it back, you learn that you don't need them and their jumbled crap. Keep up the good lessons I will recommend you to my friends. Thanks, — Don L.

Your video is great. I love the game, and now I can play it better thanks to you. I can enjoy it. Its lots of fun. Thank you so much. — M. Belair

Hi Chad, Your presentation is second to none. 100% tops. A great help to anyone how once to learn the basic of Sudoku. — Karl, Australia

Dear Chad, I want to thank you for taking the time to produce your Sudoku videos. Although I've reached the point where I can do a few hard puzzles, I don't seem to be able to push forward and do them successfully on a regular basis. Your videos have already given me a few new ways of looking at the puzzles. I am looking forward to working through the whole series. Sincerely, — Jean

Hi Chad, I'm hooked. I was seeing Sudoku everywhere & I just didn't get it. Then when I was preparing a CARE box to send to my grandson in Iraq, my daughter suggested puzzles & Sudoku was one on them. I told her I didn't get it but I never tried. She said she would show me, but I was impatient to know, so I went on line & found your Sudoku classes & now being a numbers person, I'm hooked & look forward to each lesson. Thank you, — Barb (Wesley Chapel, FL)

Just a short note to say thanks for taking the time to create the videos. Good investment for me. I'm still learning, mostly to trust my instincts and not 2nd guess myself. After watching your vids I am presently completing about 60 to 70 percent of the medium challenge puzzles. The easy ones I am pretty much trotting through, and I can sit and watch the evening news and do 3 or 4 back to back without much hesitation. These things are addictive for sure. Thanks Again, — R. Weeks

I wish to let you know how much I am enjoying your videos. I admire your professional approach and the extremely clear way you make it all come over. Very many thanks. — L. Jarman (Rugby, England)

Hi Chad, Your presentation is second to none. 100% tops. A great help to anyone how once to learn the basic of Sudoku. — Karl, Australia