A cell cannot contain a value that has been placed as the solution in any of it's buddy cells so that value can be removed as a candidate in the cell.

Most of the techniques used to solve Sudoku puzzles work by eliminating candidates until there is only a single candidate left for a particular cell, a "naked single" found by "counting", or only a single cell in a unit where a value can be placed, found by "cross hatching".In figure 1 the only possible values, "candidates", that can be placed in the blue cell are 1 and 4. The values 2 and 8 already exist in solved cells in the row, the values 3, 5 and 7 already exist in solved cells in the column and the values 9 and 6 already exist in solved cells in the box. The values 1 and 4 are the only values that have not already been used in solved buddy cells to the blue cell. The values 1 and 4 are the candidates for the blue cell.

The green cells has three candidates. The values 1, 4 and 5 are the only values that have not been used in solved buddy cells to the green cells.

The yellow cell has four candidates The values 1, 4, 7 and 8 have not yet been used in solved buddy cells to the yellow cell so it has these four candidates.

The red cell has 5 values that have not been used in solved buddy cells, so it has five candidates.

Many of the easier Sudoku puzzles can be solved by only using the values that have been placed into the puzzle and not considering candidates. ie. by using Cross Hatching and Counting.

Nearly all of the harder puzzles and most of the solving techniques implemented in this helper consider candidates for cells and work by removing candidates until there is only a single candidate left in a cell or only a single cell in a unit that has a particular candidate.

Tracking candidates when working with pencil and paper can get very difficult. Using the Sudoku Helper makes it much easier to see candidate placement and patterns and get used to looking for those patterns.

Sudoku Rules and Techniques

Sudoku Rules

Acknowledgments

I would like to acknowledge these sites referenced for strategy ideas and solving techniques.

SudokuWiki.Org by Andrew Stuart

PaulsPages.co.uk by Paul Stevens

Sudoku Rules

Cross Hatching

Counting

Naked Set

Hidden Set

Claiming

Remote Pairs

X Wing

XY Wing

Simple Coloring

Fishy Things

Unique Rectangle

X Chain

Aligned Pair Exclusion

Forcing Chains

Finned Fishy Things

Almost Locked Sets

Counting

Naked Set

Hidden Set

Claiming

Remote Pairs

X Wing

XY Wing

Simple Coloring

Fishy Things

Unique Rectangle

X Chain

Aligned Pair Exclusion

Forcing Chains

Finned Fishy Things

Almost Locked Sets

Acknowledgments

I would like to acknowledge these sites referenced for strategy ideas and solving techniques.

SudokuWiki.Org by Andrew Stuart

PaulsPages.co.uk by Paul Stevens