To complete the SUDOKU puzzle shown below left, you must place a single digit into each blank square in such a way that every row, every column, and every box contains exactly one of each of the digits 1 to 9. The solution to this puzzle is shown below right.

 The Puzzle The Solution
SUDOKU Topics
• Cell
In everything that follows, cell will be taken to mean one of the 81 squares which make up the SUDOKU puzzle.
• Unit
A unit is any of the following sets of 9 cells:-
• Any one of the rows A to I.
• Any one of the columns 1 to 9.
• Any one of the boxes which are the 3x3 sets of cells delineated by the bold outlines.

The definition of the Statement of the task could now be rewritten as:-
To complete a SUDOKU puzzle you must place a single digit into each cell in such a way that every unit contains exactly one of each of the digits 1 to 9.
• Unit Intersection  A Unit Intersection is defined as the area of a SUDOKO puzzle which is common to a box and either a row or a column. The graphic shown here demonstrates the intersection of box 7 and row H. The intersection consists of the cells H1, H2 and H3. The relationship between the candidates in the intersection, and in the two units which intersect provides the basis for a very powerful solution technique which will be discussed later.
• Candidate  When solving any but the simplest puzzle, it is necessary to insert into each cell, a list of the numbers which could possibly become the solution for that cell. These numbers are referred to as candidates. If you are solving the puzzle on paper you have no option but to enter these numbers manually, which is a tiresome and error prone activity. If you are solving the puzzle using the interactive Solve function provided by the Crossword Express program, this will be done automatically for you, allowing you to concentrate on the more challenging task of applying the advanced solution techniques which you will encounter shortly. As you progress through the solving process, you will see the number of candidates gradually diminish each time you enter a new solution number into a cell. For those hardy individuals who refuse to use candidates, the Solve functions has an option which allows you to turn off the automatic candidate assist mode.