Why? Because when a cell gets down to only one candidate, that lone number must be the answer! But don't grab your pen just yet. Great! We want to see as many of these as possible. Note that a couple of the cells have only one pencil mark. Once you have all the pencil marks filled in, your puzzle will look something like this: In fact, I used Sudoklue to create the screenshots you see on this site. My personal favorite, hands down, is "Sudoklue" ( //It does just about anything you want and is very versatile. These programs do everything from creating new puzzles for you, to acting as a virtual worksheet, complete with virtual pencil and pen, and even helping you solve them step by step. If you would like to skip using actual pen, pencil, and paper, there are several good computer programs out there for working on Sudoku puzzles. This makes it easier to see identical pairs of them later. Even if I am only writing 2 or three marks, I put them in their usual spots. I also like to always put them in in the same uniform order: 1, 2, & 3 go across the top 4, 5, & 6 across the middle, and 7, 8, & 9 along the bottom. I recommend writing the "pencil marks" very lightly. Do one cell at a time, and finish that cell before moving on. Don't go through first and do all the 1s, then the 2s, etc - because if you get distracted (if the phone rings or something), you may end up with some unfinished cells. It is very important, though, that you write all the possible candidates in a cell before moving on. You do not know what number actually goes in that cell, but you have it narrowed down to a list of candidates, and you are noting these candidates so you can see them at a glance. What you are basically doing is taking notes. Once you have filled in a cell, move on to the next, and repeat the process for all the empty cells. In other words, write all the numbers 1 through 9 except those that already appear in that same row, column, or block. Grab a pencil (so you can erase), and lightly write all the numbers that are still possible inside the cell. Look at all the numbers in that cell's row, column, and block. Here's how you do it: Find an empty cell. The idea is that you would then logically and methodically remove those marks, or candidates, one by one. In a nutshell, pencil marking is writing little numbers as a way of keeping track of the remaining possibilities, or candidates, for all the cells that are still unsolved. You have solved cells by simply scanning with your eyes.īut if you find yourself with a puzzle that is not yet complete, and you no longer see any "Open Singles" or numbers you can fill in using "Visual Elimination," then you will need to move on to Pencil Marks.Īlthough the act of making pencil marks itself is not a technique per se, it is something that is required for all of the rest of the Sudoku solving techniques. Until now, the techniques I have covered do not require any "note taking".
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