What to Rescan After You Place a Sudoku Digit

Why is a systematic rescan so important?

Placing a digit changes the puzzle's entire candidate landscape. Every time you write a number, you eliminate that digit as a possibility from 20 other cells across its row, column, and 3x3 box. If you do not check these affected areas, you will miss the next logical moves, often causing unnecessary stalls. Consistent step-by-step solving relies on this disciplined follow-up. Treating each placement as a trigger for a mini-scan turns solving from a search into a chain reaction of discoveries.

• Make it a habit: 'Place, then scan.' Never place a digit and immediately look somewhere unrelated.
• If you get stuck, retrace your last few placements and rescans; you likely missed a single.

Which three primary units must you always recheck?

The first and most critical scan is the three units that directly contain the placed cell: its row, its column, and its 3x3 box. Each of these units now has one less empty cell and one less candidate digit to place. Scan each unit completely to see if any empty cell now has only one possible candidate left, creating a Naked Single.

A common error is to check only the row and column. The box is equally vital. Because a box has only nine cells, a single placement can dramatically reduce possibilities for other digits confined to that box. Missing a box rescan is the top cause of overlooked easy moves.

• Scan the row from left to right, the column from top to bottom, and the box in a systematic pattern (e.g., left-to-right, top-to-bottom).
• Physically point your pencil or finger at each empty cell in the unit as you mentally review its possible numbers.

What are 'crossing units' and why check them?

Crossing units are the rows and columns that intersect the 3x3 box you just placed a digit in, but are not the row and column of the placed cell itself. For example, if you place a digit in the top-left cell of a box, the 'crossing units' are the middle and bottom rows of that box and the middle and right columns of that box.

Checking these is a powerful advanced rescan tactic. The placement may have forced a specific digit to have only one remaining legal position within a crossing row or column inside that shared box. This can reveal a Hidden Single. This step connects your local placement to global constraints, a core principle of candidate elimination.

How do candidate lists change, and how do I track them?

When you place a digit, you should mentally or physically erase that candidate from all cells in the same row, column, and box. The immediate effect is that some of those cells will lose their last possible candidate besides the one you just placed, creating new Naked Singles. This is why the primary unit scan is so effective.

A more subtle effect is on cells that are not in the direct units but share a row or column with a cell that just had a candidate eliminated. This can create new Hidden Singles or set up pairs and triples. While full Sudoku scanning is ongoing, the post-placement focus should be on the most affected cells in the direct units first.

What is the most common rescan mistake?

The most frequent and costly mistake is neglecting to rescan the 3x3 box that contains the newly placed digit. Players often check the row and column meticulously but then their eyes jump to a different part of the grid, skipping the box entirely. Because the box is a compact, high-interaction zone, a single placement can instantly resolve one or more other cells.

This error explains why solvers often stare at a puzzle, convinced there are no moves, when several Naked Singles are present. Developing the muscle memory to always sweep the box—looking at each of its eight other cells—will dramatically reduce your solve times and frustration.

• Verbally remind yourself: 'Row, Column, Box' after every placement until it becomes automatic.
• If you are stuck, close your eyes and mentally visualize the box of your last placement, checking each cell.