All Rules and Tricks of Sudoku Explained

Sudoku is one of the most popular games that challenge your mind and helps sharpen it. The basic rule of this game is that each column, row, and box must contain all digits from 1 to 9 and each of the digits can appear just once.

Basic Sudoku Terms:
Full House or Last Digit
This method is very easy to solve.
Please look at box #7 highlighted in Sudoku.
Hidden Singles
This term means that there is just one possible cell for a digit in a specific house and the digit is hidden among other possible digits that can be put in that cell.
Have a look at the marked box on this Sudoku and try to see which cells could contain digit one in that particular house.
Naked Singles
Buddy cells are cells that are in the same box, row or column.
Look R5C8 in this sudoku puzzle and see for yourself that the buddy cells are highlighted in grey color.
Pencil Marks
You can track possible values for a cell by using pencil marks. You sometimes do it for more than one cell, and you keep the candidates written within the cell itself. These are known as pencil marks.
Why don’t you fill in the pencil marks for R5C5? To start, it can be any digit between '1' and '9'.
The Art of Using Filters/Highlights
Highlighting is a tool that you can use for identifying complicated and simple steps. In live sudoku, you will have the highlighted menu available on every puzzle, and you are free to click on any digit between 1 to 9 with the mouse to highlight it within the puzzle. A shortcut is to press Alt, and the number you are seeking like alt + 5 will highlight the number everywhere.
Locked Candidates Type 1 (Pointing)
You should remember that if in a box, all the candidates of a specific digit, let’s assume one is confined to just one column or row, that digit, i.e., one cannot appear outside of the box in the same column or row.
Look at this example to understand this better.
Locked Candidates Type 2 (Claiming)
As you might have guessed, this method is opposite of type 1. If in a column or row, all candidates of a specific digit, let’s assume two this time are in only one box, you can eliminate that digit, i.e., two from all other cells in that box.
Naked Pairs / Twins
In case there are two cells in a house, column, row or box that have just two candidates, you can eliminate these two candidates from all other cells in that house. The pencil marks for row #8 are marked here. If you take a look at R8C7 or R8C8, both are in the same row and they have just two candidates each.
Naked Triplets or Triple
In case there are three cells in a house (which means row, column or box) which have three same candidates, you can eliminate those 3 from all other cells in that particular house. It is not vital that all three cells must contain 3 candidates, but these three cells must contain the three candidates between them. Also, remember that there can’t be more than 3 candidates in each cell. To get more clarity, please check R3C5, R2C4, and R2C6 in the box where 3 cell candidates are 9, 7 or 5.
Hidden Pair
If there are two cells in a house where there are two candidates which appear nowhere outside these cells in the same house, those candidates should be placed in the two cells, and hence all other candidates will be eliminated automatically. Have a look at column C5, here, R7C5 and R2C5 both have 5 and 2 as candidates, and these two digits are not candidates of any other cell in this column.
X Wing
If you have a look at row number 4 and row number 2 in this puzzle, you will see that cells which are highlighted in red can contain 8 in those two rows.
Swordfish
This works very like X wing, but it has three base sets, not just two.
Have a look at the following puzzle, and you will be able to see that 6 is the fish digit while rows R9, R6 and R1 are base sets. Y or XY Wing
This is another advanced technique that helps in eliminating candidates, where you start by finding a cell that has only two candidates known as the pivot. These candidates will be called X and Y. In this sample, R5C5 is the pivot where X is 6 and Y is 9.
It is hoped that you now have a better understanding of how to play Sudoku and you’ll use these rules when and tricks while playing sudoku online the next time.