Pencil marks

When naked singles and hidden singles run out, holding everything in your head stops working. From here on you write the digits each empty cell can still take (its candidates) in small print. Every technique that follows becomes visible in these notes.

What candidates are

A cell's candidates are the digits not yet in its row, column or box. A naked single is a cell with one candidate; a hidden single is a digit that appears as a candidate only once in a line. Pencil marks simply lay this information out where you can see it.

4597281637213683681791267483587541664318579482356712379256812418657239249249

Every empty cell of box 9 lists the digits it can still take. Each digit has a fixed spot in the cell, laid out like the box (1 top left, 9 bottom right). Play this board →

Each candidate has a fixed spot in the cell. On a 9×9 board, 1 sits top left, 5 in the middle, 9 bottom right. With fixed spots you can ask “where is 7 in this box?” and just check the same corner of each cell.

When to start

Don't fill every cell at the start. Pencilling 50 empty cells takes a long time and invites mistakes.
Go as far as you can with singles first. With fewer empty cells there are fewer candidates to write.
Start near where you are stuck, especially lines and boxes with three or four empty cells. A few notes are often enough to reveal the next technique.

Erasing matters more

When you place a digit, remove it from every note in the same row, column and box. Miss one and a digit that is already blocked stays as a candidate and leads you to a wrong conclusion. Most mistakes on note-heavy boards come from this.

Everydoku removes the digit from those notes for you when you place it. If you want an erased note back, press undo.

What notes reveal

With notes written, patterns start to stand out. Each one is the subject of a later article.

In a box, a digit's candidates all sit in one row or column → locked candidates
In a line, two cells have the same two candidates and nothing else → naked pair
In a line, two digits appear only in the same two cells → hidden pair
In two rows, a digit appears only in the same two columns → X-Wing

Notes in Everydoku

Turn on Notes and tap digits to write them as candidates; tap again to erase. On a computer, the space bar toggles notes.
Tap a cell that holds a digit and that digit lights up across the board, in notes too. Tap a 7 and you can see at a glance where 7 can still go.
On 12×12 and 16×16 the cells are small, so notes are drawn as dots, and the digits you wrote appear large in the notes bar beside the board (below it on phones).
Big boards also have Auto notes, which fills in the current candidates of the selected cell. It never fills the whole board at once, on purpose: deciding where to look is still your job.

A sign your notes went wrong: if an empty cell has no candidates left, a digit you placed earlier is wrong. Everydoku shades such a dead-end cell red so you notice right away.