# Sudoku Instructions - candidate reduction methods

Please note: This section will be more relevant to you and easier to follow when
you have acquired the Sudoku Instructions program and the free course in sudoku.

In some sudoku puzzles there are too many possible digits in each square - there
are too many candidate digits. To solve the problem it may be necessary to eliminate
some of the candidates - to perform candidate reduction.

## Examples

Here are some examples of how Sudoku Instructions program can find candidate digits
that can be removed.

All methods are based solely on logic. Sudoku Instructions program will explain clearly
how these methods work.

Sudoku Instructions will show the candidate digits that the methods use in blue color.
The candidate digits, which can be eliminated, are shown in red color.

## Locked candidate:

In row 3 candidate digit 3 (blue digits) is only present inside box L. In this box
digit 3 has to be in row 3. It is "locked" in the row. Therefore, you can remove
3 as a candidate in the other rows inside box L (red digits).

You can do this in the program by simply clicking on "Perfom This Reduction".

## Hidden candidates (pairs):

In box J above candidate digits 1 and 5 (blue color) are isolated "hidden" candidates
in a pair (or as twins). They are called hidden because they appear hidden among
other candidates - in this case 8 (red color) in squares A1 and B1. Candidate digits
1 and 5 are isolated because they are not present anywhere else in the box. 1 and
5 must therefore be in these two squares. So, you can remove other candidates (here
8 - shown in red) from the squares.

You can do this in the program by clicking "Perform This Reduction".

Hidden candidates in the form of triples or quads can be identified in the same way
. These somewhat trickier reduction methods are explained in the program.

## Naked candidates (pairs):

In row 1 candidate digits 1 and 5 (blue digits) form a naked pair (naked twins) in
squares A1 and B1. They are called "naked" because they are alone in the two squares.
These digits must therefore be in those two squares. You can remove candidate 5 from
other squares in row 1 (red digits).

You can do this in the program by clicking the "Perform This Reduction".

Naked candidates in the form of triples or quads can be identified in the same way
. These somewhat trickier reduction methods are explained in the program.

## X-wing, swordfish, jellyfish:

The candidate digits 1 (blue color) form an X-wing, because in row 6 and 8 candidate
digit 1 is only present in column D and F. So, in these columns digit 1 must be the
rows 6 and 8. In one row 1 must be in column D, in the other row 1 must be in column
F. So, you can remove candidate digit 1 (red color) from the other rows in column
D and F.

You can do this in the program by clicking the "Perform This Reduction".

Swordfish (one candidate digit in just 3 rows or columns) and jellyfish (one candidate
digit in only 4 rows or columns) can be identified in the same way. These difficult
reduction methods are explained in the program.

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