Sudoku Lesson II - Projecting the numbers

The easiest and most basic technique toward solving Sudoku puzzles is figuring out when a number must go in a box. One way to do this is to choose a number, then cycle thorough all the squares to see if the chosen number has been entered or not in the target square. If not, then project the number down rows and columns from other squares to exclude boxes in the target square. When all projections are made, if there is only one box where the number can go, write it in:

Here we can see that the number 1 fits in box (9, 9), then (8, 1,), then (3, 3).

If there are only two boxes in a square where a number can go, mark those boxes:

In this picture, the number 2 can go in boxes (1, 7) and (2, 9). If there are more than two boxes where a number can go, move on to the next target square and try again. Do not be tempted to mark three or more boxes. we need to save that kind of activity for later. Don't worry, I'll tell you when you can do that -- you gotta trust me for now.