Basics

Sudoku

You wouldn't be reading this if you'd never seen a Sudoku, but here's one anyway —

Sudoku Grid. There are 81 Cells in the Grid. The (horizontal) Rows are numbered 1 to 9 from top to bottom. The (vertical) Columns are numbered 1 to 9 from left to right. There are nine 3 x 3 Blocks. "House" is the generic term for a Row, a Column, or a Block.

The Sudoku is specified by the Clues, which are the Big Numbers (BigNums) initially supplied in some of the Cells. The remaining Cells are empty, and you have to fill them in with BigNums to satisfy three conditions:

• every Row must contain the digits 1 to 9;
• every Column must contain the digits 1 to 9;
• every Block must contain the digits 1 to 9.

Notice that that implies that there are no duplicate digits in any one House.

A Sudoku is guaranteed to have one single solution.

Two Sudokus are different if neither is a shuffled or digit-permuted clone of the other. Different Sudokus lead you to a different succession of logical traps. It has been proved that there is an amazingly large number of different Sudokus. You won't run out.

Depending on the Clues given, some Sudokus are easy to solve, and some are hard. Counting the number of Clues given will not tell you how hard a Sudoku is.

In a way, a Sudoku looks simple. But solving one can turn out to be very difficult. In response to this fact, people have cooperated in gradually developing two major Strategies for solving Sudokus. The more sophisticated of these two Strategies is composed of a rather large number of different Tactics (see Strategies & Tactics).

Because of the complexity of these Tactics, it's helpful to have some terms to describe the anatomy of the Grid, and that's what we'll look at next.

 

Terminology

The location of a Cell is specified by its Row and Column coordinates.
R3C2 means the Cell that's in Row 3 and Column 2.

RpCq notation. The green Cell is R3C2. The blue Cell is R8C4. The four orange Cells are R46C59, which means R4C5, R4C9, R6C5, and R6C9. You will also occasionally see the notation R257 meaning Rows 2, 5, and 7. Similarly, C34 means Columns 3 and 4.

Rows are grouped into Bands:

• Upper Band = Rows 1,2,3
• Middle Band = Rows 4,5,6
• Lower Band = Rows 7,8,9

Band. The three coloured Rows together form the Upper Band. Notice that a Band also contains three Blocks.

Columns are grouped into Towers:

• Left Tower = Columns 1,2,3
• Center Tower = Columns 4,5,6
• Right Tower = Columns 7,8,9

Tower. The three coloured Columns together form the Center Tower. Notice that a Tower also contains three Blocks.

A Block is the intersection of a Band and a Tower, and it is named on that basis.

Block names. The Upper Right Block is shown in blue. The Middle Center Block is shown in yellow. The Lower Left Block is shown in purple.

A Bandit is a horizontal sequence of three Cells which lies entirely within one Row and one Block (it's the intersection of a Row and a Block).

Bandit. A Row is composed of three Bandits. A Block is composed of three Bandits.

A Twit is a vertical sequence of three Cells which lies entirely within one Column and one Block (it's the intersection of a Column and a Block).

Twit. A Column is composed of three Twits. A Block is composed of three Twits.

A Bandit is a "little Band".  A Twit is a "little Tower".  (Alternatively, you can call a Twit a Turret.)

 

Any More Basic Material?

Yes:  read the page on Candie Markup before reading the pages on Tactics that fall under the Bookkeeping Strategy.

 

Sources of Sudokus

Newspaper Sudokus are very small. If they're easy, then they can be solved by the Scanning Strategy, and the size of the Grid is no problem.

But medium and hard Sudokus are solved by the Bookkeeping Strategy, which requires the entry of Candie Markup (pencil marks) in the empty Cells, and for that, you need a lot of room. At a minimum, a usable Grid has to be 6 inch x 6 inch (15cm x 15cm), but 8 inch x 8 inch (20cm x 20cm) is even better.

Fortunately, you can get free large-Grid Sudokus puzzles online, and I recommend that you do that (see Links).

 

Tools

To solve Sudokus, you'll need some hardware. And to justify these needs, I have to use some terms that you might not know yet — but they'll all make sense soon enough as you begin to work your way through the material on this site (and on other Sudoku sites).

You'll want a fine- or medium-point ballpoint pen for entering Candie Markup (this markup is referred to as "pencil marks" on other sites, but you'll be much happier if you enter these small numbers in ink).

You'll need a thick-lead pencil for killing (cancelling) candies and for entering the final Big Number in a Cell.

And a thin-lead pencil is good for entering Twin Tagging (useful for tagging Target Chains).

A long eraser stick is useful for the (rare) mistakes you'll make.

Coloured-lead automatic pencils are available in office supply stores. It's useful to have from two to four different colours. The point is that a light pastel colour stands out, but it doesn't get confused with the main markup (the candies) on the Grid. You'll find it helpful to border-colour the Cells of a Unique Rectangle and to circle its Heroes in colour.  And for an Attack Chain, writing in Cell labels A,B,C,... in light colour gives a result that's clear but unobtrusive. You might be dubious about this, but if you try it, you'll like it.

If you get to the point that you want to try a Smart Fork, you'll need some sort of photocopying mechanism — photocopier, fax-machine copier, or scanner-and-printer — to make two photocopies of your Twin-Tagged Sudoku.

You'll want a comfortable reclining chair. A pillow and a clipboard to support the Sudoku sheet. A flask of açaí berry juice.

 

Help

In the beginning, you'll want help.

The web is loaded with help; how else would I have learned how to do Sudokus? See Links for a bunch of good sites.

On this site, you'll want to read the (short) Candie Markup page and then skim-read the Strategies & Tactics page to get an overview of the process of Sudoku solving. After that, you can dive into the various Tactics pages, which are organized in groups in the navigation bar at the top of every page.

Some of the Tactics are sufficiently complex that it was necessary to provide background material on separate pages. In particular, the Deadly Patterns page gives the needed background for the Uniqueness Tactics, and the Twin Tagging page introduces a notation that's useful for some of the Network Tactics.

If you experience any problems viewing or printing the pages on this site, there may be a solution:  check out Viewing This Site to see if there's anything in there that will help.