Spot easy-to-find 3x3s in blocks. A 3x3 happens when there are 3 squares that can only be filled by 3 potential numbers. When these 3 squares are in a block, they’re normally especially easy to spot. In the above example, you’ll see that the top left block has 3 open squares that could only be filled by a 2, 3, or 6. Additionally, we can eliminate the 2 as a potential candidate for the middle square, since there’s a 2 located in that same row. Even better, we can eliminate 6 on the right-most square, since there’s a 6 in that column. These eliminations will make this 3x3 even easier to solve in the future.

Look for less clear 3x3s in columns. Vertical columns are also a great place to spot 3x3s—each column and row needs to have every number from 1 to 9, not just each block. Remember to always keep columns in mind when solving a Sudoku. In this example, notice that 2, 5, and 9 are the potential candidates for the missing squares in the vertical column. We can also eliminate 5 from the middle square, since there is another 5 in that row already.