**A Sudoku puzzle is a grid of nine by nine squares or cells that have been
subdivided into nine subgrids of three by three cells which are called regions,
other terms include “boxes”, “blocks”, and the like
when referring to the standard variation. There are some variations which
include the use of larger or smaller squares than the nine by nine classic
grid, irregular divisions of the overall grid, and additional restrictions
on the placement of the numbers. **

The attraction of this puzzle is that the rules are simple, yet the line of
reasoning required to find the solution may be complex. Sudoku is not a mathematical
or arithmetical puzzle. It works just as well if the numbers are replaced
with letters or some other symbols, but numbers work best. On the other hand,
if we look into Sudoku a little more deeply, we may as well find some mathematical
ideas lurking in the background.

It is usually recommended by many experts as an exercise in logical reasoning.
Studies show that playing Sudoku improves memory and mind clarity. The puzzles
are often available free from published sources and some are custom-generated
using software.

**There is just one basic rule in solving Sudoku puzzle: Fill all empty
squares so that the numbers 1 to 9 appears once in each row, column, and three
by three box. As the name "Sudoku" comes from the Japanese abbreviation,
"Suuji wa dokushin ni kagiru" which means "the digits must
remain single". This name is a registered trademark of a Japanese company
- Nikoli Co. Ltd in Japan.**

**In each Sudoku puzzle, several digits have already been entered or are
given as clues; these numbers cannot be changed. The degree of difficulty
of the puzzle is determined by which numbers the puzzle maker chooses to reveal
as clues. In an easy puzzle, a lot of numbers are shown and it is pretty straightforward
to figure out what the rest of the numbers are. **

**In a harder puzzle, few numbers are shown and you have to do a lot of
investigation to figure out which numbers are missing. Difficult puzzles require
solvers to look for contingencies in the puzzle. Contingencies usually require
a solver to mark-up the puzzle with all the different options in each square.
Sometimes there will be no way to eliminate a possibility for a square and
you will have to choose between two or more numbers. **

**If this is the case, it is important to remember where you made the choice
to choose a number and backtrack if your original choice was not correct.
One does not need to guess. Every Sudoku puzzle can be solved logically. The
best way to start is by looking for clues. Most people start by looking for
numbers that occur frequently in the initial puzzle.**

**For example, you have a lot of 7's in the initial puzzle. Look for the
region where there is no 7. Can you rule out places where a 7 might go? Look
for 7's in other columns and rows that can help you eliminate where the 7
might go in that box. Remember, a 7 can't appear more than once in any region,
row, or column. If there is a 7 in column's 1 and 2, then there can't be a
7 anywhere else in either of those columns. You know then that whatever leftmost
region that is missing a 7 must have it go in column 3. **

**If you can eliminate all the possibilities in that box except for 1 square,
you've got it down! Once you've finished looking at all the regions make sure
to fill in the 1-9's in rows and columns that have most of the numbers in
them. If there are only two numbers missing in a column or row you can usually
use a process of elimination to figure out where to put in the last two numbers.**

**Scan the column or row where the missing numbers two are, if you see or
eliminate one of possibilities based on the other row or column you can now
complete the line you were working on. Instead of guessing which can lead
you but to nothing, several common methods to solve this puzzle are usually
used. **

**To give you an overview, the method is a combination of three processes:
scanning, marking up, and analysing. In general, one should always use one
method before using the next more complex ones. Then, one can try again with
the simpler method, and so forth. Remember that no matter how well or bad
you are at solving Sudoku, this puzzle will surely give you a good mental
workout. In short, Sudoku puzzle is as good as it gets.**