Number grid

How it works

What you get from it

A quiet, steady kind of focus. Nothing here is luck: every digit follows from the digits already on the board, so the grid rewards patience rather than speed.

It also teaches you. Easy grids need only the two simplest moves. Harder ones bring in named techniques, one at a time, and the hint shows each on your own board, so you learn to spot them yourself.

Rules

The grid has nine rows, nine columns and nine 3×3 boxes. Fill every empty square so that each row, each column and each box holds the numbers 1 to 9 once. The bold numbers are given and stay put. There is exactly one way to finish the grid.

Every grid is checked before it ships: one solution, reachable step by step with the techniques below.

How to play in Rather

Tap a square, then a number on the keypad. The square turns butter yellow and its row, column and box are washed, so you see what it can see. Tap the same number again to clear it.

Turn on Notes to pencil small candidates into a square, up to nine of them. Placing a number clears that square’s notes. Undo steps back through every change, notes included.

Where to start

Pick a number, say 8, and pick a box. Look along the rows and columns that cross it: every 8 already there rules out a line of the box. When only one square is left, the 8 goes there. This is a hidden single, and it opens most easy grids.

Then look at single squares. If a square’s row, column and box already hold eight different numbers, the ninth is the only one left. That is a naked single.

Medium grids add pointing (a box’s 2s all sit on one line, so the rest of that line has none), naked pairs and hidden pairs. Hard ones add naked triples and the X-wing.

A good habit: after you place a number, look at the box, row and column it just filled. A new number often opens another single right next to it.

When you’re stuck

Tap Hint. If a number is wrong, it clears it first. Otherwise it places the next number you could work out from your board and says why: “8 fits nowhere else in this box.” The cells it read are outlined.

Errors underlines the numbers that differ from the solution, only when you ask. If two equal numbers share a row, column or box, it names the rule they break.

Sizes

• Easy 9×9 Hidden and naked singles are enough.
• Medium 9×9 Adds pointing, box-line, naked and hidden pairs.
• Hard 9×9 Adds naked triples and X-wings.

Questions

Do I ever have to guess?

No. Each grid is rated by the techniques it needs, and every step follows from what is on the board.

Does the hint read my notes?

No. It works from the numbers you have placed, so your notes are yours.

Does a finished box light up?

No. Nothing is marked until the whole grid is solved.