What carries over
The rules are identical: each digit once in every row, column and box. So is the way of thinking. You already scan a digit across the board, spot the last empty cell in a box, and notice a cell that has run out of options.
Those are hidden singles and naked singles, and they are still the heart of the nine-by-nine. Easy puzzles on the big grid use nothing else. You aren’t learning a new game.
What changes
- More to fill. The daily Mini Sudoku leaves 21 empty cells. The daily Sudoku leaves 49.
- More to check. A cell on the small grid shares a row, column or box with 12 others. On the big grid it is 20, and a cell needs to see eight different digits, not five, before it is settled.
- Square boxes. Rows and columns behave alike, and each crosses three boxes.
- Longer waits. On a six-by-six, something is always available. On a nine-by-nine you can scan every digit and find nothing, and that is normal.
The biggest change is that you can no longer keep the whole board in your head. That is what notes are for.
The two-row trick needs two digits
On the small grid, a digit in one box tells you which row it takes in the box beside it. On the big grid three boxes sit side by side across three rows, so one copy of a digit leaves two rows open.
You need two. When two boxes in a band have the digit in different rows, the third box must have it in the remaining row. On the right, the 7s in rows 1 and 2 push the middle box’s 7 into row 3, and the 7 in column 4 then leaves a single cell.
This is the first thing to practice. It is the same move you already make, with one more step.
Habits to drop
- Solving entirely in your head. It works for 21 cells. For 49 it leads to mistakes.
- Expecting a naked single at every turn. They are much rarer when a cell has nine possible digits. Lean on scanning for hidden singles.
- Rushing. A wrong digit on a nine-by-nine can stay hidden for 20 moves before it causes a conflict.
What to learn, in order
You don’t need everything at once. Each of these has its own page with a worked example.
- How to start a puzzle Scanning on the big grid, one digit at a time.
- Hidden singles In rows and columns as well as boxes.
- Naked singles Counting to nine in a single cell.
- Naked pairs and triples Your first use for notes.
- Pointing pairs For the day a medium puzzle stalls.
A plan for the first week
Keep playing the daily Mini Sudoku as a warm-up, then open the daily Sudoku straight afterwards while the scanning habit is fresh. Don’t time yourself at first. Aim to finish without a conflict.
Expect it to take several times as long as the small grid, and to get quicker within days. If you leave partway through, your entries are saved. When you meet a word you don’t know, the glossary has it.