Sudoku Strategies and Tactics

Have you gotten good at solving Sudokus by straight Scanning?  Have you progressed to harder Sudokus where Scanning isn't enough and you sometimes get stuck?  Are you interested in learning how to use Candie Markup (pencil marks), or how to use it more efficiently?  Then this site was written for you.

Fundamental background information is provided in Basics and Strategies, and you should skim through these quickly to see if they clarify any ideas you need to know.

There is a Glossary that covers the terminology used on this site.  (Sudoku terminology is not really standardized, and some new terms are used here.)

The emphasis in these pages is on Sudoku-solving Tactics which have frequently occurring, easy-to-recognize patterns and which often have big payoffs.

Background for the Bookkeeping (pencil-mark) Strategy is given in Candie Markup. Based on that, the basic Bookkeeping Tactics are discussed, including Locked Sets, Claims, and 2-Fish.

Uniqueness Tactics are thoroughly discussed, with lots of examples given in the pages on Unique Rectangles and BUG+n. The logic behind these Tactics is subtle, but it's worth understanding if you want to be confident that what you're doing makes sense. A full discussion of the issues involved is given in the page on Deadly Patterns.

The two most powerful components of a Sudoku Grid are Pairs and Twins. Pairs are visually obvious, but Twins are not. The technique of Twin Tagging is introduced to make Twins and the relations between them visible. Twin Tagging makes it easy to develop the notion of a Worthy Target (a Fault Point or a Fault Line):  attacking such a Target may crack the entire Sudoku, or at least clean up the Grid substantially.

A Chain, which is sometimes necessary to solve a hard Sudoku, is possibly the most satisfying of all Sudoku Tactics because instead of looking for a set pattern, you construct your own pattern to crack a Worthy Target that you have previously located.  In the page on Chains, the ideas are first built up gradually, and then concrete full-Grid examples are given.

For a really hard Sudoku which has thwarted you but nevertheless happens to contain a high-Degree Fault Point, the Smart Fork Tactic can be applied if you refuse to give up.

Finally, Diabolical Sudokus are discussed. No magic technique is presented for handling these, but some brief remarks on the current state of the art are given in The Quest.

As of 2011 January 7, the Smart Fork page has been completely rewritten to provide a more accurate presentation.  The changes involved there gave rise to modifications of several other pages, the Chains page in particular.

No-one's thoughts on Sudoku solving ever really match your own, so you should read over the ideas of as many different people as possible. The Links page will give you a list of references to some helpful Sudoku web sites.

Read Viewing This Site if you are experiencing any display problems; in particular, see that page for remarks on using the <−  −> −> <− buttons to control the content width, and the +   buttons for controlling the text size. And be sure to read through the remarks on Text Size and Printing on that page before printing out any of the pages on this site.


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This page was last updated on 2011 January 7.

The home page for this site is




Why Links Here?

Roving page analyzers require the presence of physical links on a web page.

The navigation bar at the top works of course, but it is Javascript driven, which means that it contains no physical links.

So this is a complete list of links for the rovers to use:

Explanatory Material —
Candie Markup
Twin Tagging
Deadly Patterns
Diabolical Sudokus

Remarks —
Viewing This Site

Scanning Tactics —
Nearly Full House
Row/Column-on-Block Shadowing
Block/Column-on-Row Shadowing
Block/Row-on-Column Shadowing
Only Digit

Local Tactics —
Locked Single and Hidden Locked Single
Locked Pair, Trio,...
Hidden Locked Pair, Trio,...

Grid-Sweep Tactics —
Other Fish

Uniqueness Tactics —
Unique Rectangle

Network Tactics —
Smart Fork

Silver Bullet Tactics —
The Quest