Mirrored from Sudopedia, the Free Sudoku Reference Guide
Various diagrams and notations are commonly used by the Sudoku community. As such, these diagrams and notations can be found sprinkled all over Sudopedia. A good part of these are developed so that communication becomes possible in textbased forums, and also for compactness reasons. This article is intended to guide the reader to some of these communications tools.
Although this article refers to several solving techniques, this article merely use them to illustrate diagrams and notations that are used. Since the aim of this article is not to describe these techniques, readers are advised to refer to the various other Sudopedia pages that describe them in more detail.
Contents 
In a Sudoku, you are presented with a starting grid with some digits already given in some cells. These digits are usually referred to as givens. Your task is to fill up the remainder of this grid, as described in the introduction. An example grid is shown below. (This grid is rated easy and can be solved using singles alone.)
However, in the Sudoku Forums, it is often inconvenient (or sometimes impossible) to upload graphics (e.g. GIFs, JPGs, PNGs). So an alternative textbased format for the same grid is as follows.
.... 3 1 .. . 4. 6 9 . . .. . .2 . . . . 8. . 5. 4 . :  : . . .. . .. . 5 . . 6. . .. 1 7 8 . 7. 3 .. . . :  : 5 9 .7 . .. . 6 6 . .. . 3. 5 . . . .1 . .. . 2 ''''
The above format allows easy visualization of the grid, but it takes up a fair amount of screen estate. Sometimes, people want to describe many starting grids in one go, say, as examples that can be solved using a certain solving technique. For such usage, the Sudoku grid is often presented as an 81digit string. In this format, the digit 0 is used to denote an empty cell and all the values of the cells are concatenated into one long string. So the 81digit string of the example grid is shown below.
310004069000000200008005040000000005006000017807030000590700006600003050000100002
Most good Sudoku Programs are able to accept such 81digit strings as input, usually through their paste function. These programs usually also allow users to export the grid in any of the above textbased formats via their copy function.
Unless a given Sudoku grid can be solved using singles alone, most Sudoku players will need to maintain pencilmarks while solving the puzzle. Basically, the pencilmarks indicate the possible candidate digits for each empty cell. Practically all solving strategies operate by eliminating candidates from cells until only each cell has only one candidate remaining. Cells with only one possible candidate digit can be immediately placed with that digit, in what is known as the Naked Single technique. Due to the presentation of most Sudoku programs, the givens and the placed digits are often referred to as big numbers while the pencilmarked candidates are often referred to as small numbers. The same example grid with pencilmarks is shown in the graphic below.
Again, for communicating in textbased forums, the textbased format for the grid with pencilmarks is shown below. It is sometimes called the candidate grid, and it has no notion of big numbers and small numbers.
....  3 1 25  28 278 4  578 6 9   479 4567 459  3689 16789 16789  2 378 138   279 267 8  2369 12679 5  137 4 13  :  :  1249 234 12349  24689 1246789 126789  34689 2389 5   249 2345 6  24589 24589 289  3489 1 7   8 245 7  24569 3 1269  469 29 4  :  :  5 9 1234  7 248 28  1348 38 6   6 2478 124  2489 2489 3  14789 5 148   47 3478 34  1 45689 689  34789 3789 2  ''''
Most good Sudoku programs can also import and export the textual candidate grid format.
Sudoku literature often makes references to particular cells, and the preferred notation is rncn. For example, r1c2 refers to the cell at row 1 (from the top) and column 2 (from the left). This is consistent with matrix notation in mathematics.
An alternate notation that is sometimes seen is the k9 notation, which uses a letter to denote the row and a number to denote the column. However, the k9 notation often causes confusion regarding whether the letter or the number denotes the column, and whether the lettering goes from top to bottom or bottom to top. Therefore, most Sudoku literature has adopted the rncn format.
When we say that we can place a digit 3 in the cell at row 1 and column 2, we can write r1c2=3. On the other hand, when we say that the cell at row 1 and column 2 cannot contain the digit 3, or in other words, we eliminate 3 from the cell at row 1 and column 2, we can write r1c2<>3. (It appears that BASIC programmers have first propagated this notation, for the notequals operator in BASIC is <>. Most other programming languages use != as the notequals operator.)
Occasionally, we want to refer to groups of cells, as illustrated by the diagram below.
This diagram illustrates the Pointing Pair technique (click on that link for more). We want to reference both r1c8 and r3c8 (marked yellow), so we can use r13c8 to reference them. Similarly, to say that we are eliminating the digit 4 from both r4c8 and r6c8 (marked red), we can simply write r46c8<>4.
Similarly, we can write r13c24 when referring to four cells, r135c24 when referring to six cells. These are typically seen when describing deadly patterns, such as Unique Rectangles.
We use the Pointing Pair example again to illustrate the problem of communicating in textbased forums.
Usually, a pencilmark grid is used to describe such eliminations. Typically, a *
is prefixed or appended to the cells in question to indicate the cause of the elimination, and a 
is prefixed or appended to indicate the cell where the eliminations takes place. The result is shown below.
....  2 56789 578  34569 345789 456789 1 *467 679   1 4 57  569 2 5679  679 8 3   679 6789 3  469 1 46789  5 *2467 2679  :  :  3457 12357 2457  34569 34589 45689  46789 12467 126789  34 123 6  7 3489 489  489 5 1289   8 57 9  2 45 1  3 467 67  :  :  45679 56789 4578  459 4579 3  2 167 15678   45679 5679 1  8 4579 2  67 3 567   357 23578 2578  1 6 57  78 9 4  ''''
In this example, the logic behind the eliminations involves only a single digit, the digit 4. Thus some people choose to communicate the same message by showing only the cells with 4 as a candidate, like the one shown below. This kind of grid tend to be much easier to read than the grid showing the entire pencilmarks.
....  . . .  4 4 4  . *4 .   . . .  . . .  . . .   . . .  4 . 4  . *4 .  :  :  4 . 4  4 4 4  4 4 .   4 . .  . 4 4  4 . .   . . .  . 4 .  . 4 .  :  :  4 . 4  4 4 .  . . .   4 . .  . 4 .  . . .   . . .  . . .  . . .  ''''
Note: Beginning Sudoku players may choose to skip the remainder of this section.
In more advanced techniques such as Uniqueness Tests and Bivalue Universal Graves, more elaborate markings may be used. The following example comes from Uniqueness Test.
In such techniques, the +
is used to indicate that the candidates after the +
sign are surplus candidates. Also, sometimes the 
sign is not merely prefixed or appended, but placed in the middle of the candidates. In this case, all candidates after the 
sign are eliminated. The Unique Rectangle causing the elimination is marked with *
.
....  6 9 3  7 4 8  25 25 1   1 4 5  69 2 3  69 7 8   7 2 8  1 5 69  3 69 4  :  :  35 1357 6  389 178 2  589 4 579   4 137 9  368 1678 5  268 268 27   8 57 2  469 679 469  1 569 3  :  :  2 58 1  459 89 49  7 3 6  *59 568 4  568 3 7  2589 1 *59+2 *59 3 3568 7  2 68 1  4 589 *59  ''''
For more complex techniques where the *
symbol is not sufficient to indicate the cause of the eliminations, or when illustrating multiple techniques causing multiple eliminations in a single diagram, other symbols such as #
and @
can be used.
A number of solving techniques perform elimination based on the patterns for a particular digit, and hence called singledigit techniques. In order to highlight the concept without getting lost in the large number of candidates in a typical grid, the digit is usually generically referred to as X, and the community has also developed a kind of concept diagram. In such a diagram, each cell contains a symbol:
X

or /
.
or no symbol
*
Note that in such diagrams, 
and *
have different meanings from those seen in the pencilmark grid elimination diagrams. An example below:
....  X X X  * * *  * * *                :  :             :  :             ''''
This is the Pointing Triple technique, and it says that if the candidates for X in a box is confined to a row, then X can be eliminated from the cells in the row that are not in the box. (The logic is exactly the same with Pointing Pair.)
Some people choose to show such concept diagrams using an arbitrary digit instead of the symbol X. Usually, the digit 1 is used.
The fishrelated techniques is a large class of singledigit techniques. Such singledigit concept diagrams have been further specialized to fish diagrams, by introducing a special symbol F
for fins.
Such singledigit diagrams have been extended for multiple digits, but with an important difference. A cell may now contain multiple digits, but the semantics may mean that either the cell contains exactly these candidates as indicated, or the candidates of the cell is a subset of the indicated digits. Also, the eliminations may be for one or more digits, this will be stated in the accompanying description. For example:
....  XYZ XYZ XYZ  * * *  * * *   * * *  . . .  . . .   * * *  . . .  . . .  :  :  . . .  . . .  . . .   . . .  . . .  . . .   . . .  . . .  . . .  :  :  . . .  . . .  . . .   . . .  . . .  . . .   . . .  . . .  . . .  ''''
This diagram illustrates the Naked Triple technique. It means that if we can find a boxrow such that the candidates of each cell of the boxrow is a subset of {X, Y, Z}, then we can eliminate X, Y and Z from the remainder of the row and the box containing the boxrow.
Another example:
....  . . .  . . .  . . .   . XY .  . XZ .  . . .   . . .  . . .  . . .  :  :  . . .  . . .  . . .   . YZ .  . * .  . . .   . . .  . . .  . . .  :  :  . . .  . . .  . . .   . . .  . . .  . . .   . . .  . . .  . . .  ''''
This is the XYWing technique, and the elimination is only for Z. Here, the three cells should contain exactly {X, Y}, {X, Z} and {Y, Z} as their respective candidates in order for this technique to be meaningful.
Note: Beginning Sudoku players may choose to skip this section.
Chains and loops are used in advanced solving techniques, and these have a set of notations. See the following articles for more information: