1 of 2
You know a 4 is in one of two cells in this box. Which kind of mark records that?
Level 1 · The basics
How to use sudoku pencil marks
Most variant solves are won with notes. Solvers use two kinds, and every serious app, including this one, follows the same convention.
Pencil marks are small notes you write in a cell before you are sure of a digit. Solvers use two kinds. Corner marks say which cells in a box a digit can go in, and center marks say which digits one cell can be.
Most variant puzzles are too hard to hold in your head. Clean marks show pairs and triples at a glance and make it easy to see what a newly placed digit changes.
How setters write it
The same rule, worded the ways you’ll see it on puzzles.
- Corner marks: small digits in the corners of a cell.
- Center marks: small digits in the middle of a cell.
Worth memorizing
- Corner marks say “this digit goes in one of these cells”. Put a 4 in the corner of the two cells in a box where 4 can go.
- Center marks say “this cell is one of these digits”. Write 358 in the center when a cell can only be 3, 5 or 8.
- In this app, hold Shift for corner marks, Ctrl (or ⌘) for center marks, and both for colors.
Solving tips
Corner marks track positions
Put a small 4 in the corners of the two cells in a box where 4 can go. When one of those cells gets another digit, the 4 goes in the other one.
Center marks track candidates
Write 358 in the center of a cell that can only be 3, 5 or 8. When a 5 lands elsewhere in its row, erase the 5 and the cell is down to 3 or 8.
Mark only two or three spots
Corner-mark a digit only when it has two or three possible cells in a box. Marking every option fills the grid with notes and hides the useful patterns.
Look for matching center pairs
If two cells in a row both show 27 in the center, they hold the 2 and the 7 between them. Remove 2 and 7 from every other cell in that row.
A worked example: marking one box
We name cells by row and column, so row 1, column 5 is r1c5. The top-left box is empty except for 1, 2 and 3 across its middle row. Outside the box, row 1 has a 5 at r1c5 and a 6 at r1c8, row 3 has a 4 at r3c7, column 1 has a 4 at r6c1, and column 3 has a 6 at r7c3.
- Find the 4 first. Row 3 and column 1 already have a 4, and the middle row is full. So the box's 4 can only be r1c2 or r1c3. That's two cells, so put a small 4 in the corner of each. This is Snyder notation at work.
- Both corner 4s sit in row 1. The box's 4 is in row 1 either way, so no other cell in row 1 can be 4. The corner marks show that removal at a glance.
- Now the 6. Row 1 has a 6 and column 3 has a 6, so the box's 6 goes in r3c1 or r3c2. Corner-mark both. The 5 can't go in row 1, so it has three spots in row 3 of the box. Mark those too if you use the three-cell version of Snyder.
- Switch to a center mark for r1c1. The box rules out 1, 2 and 3. The row rules out 5 and 6. The column rules out 4. Write 789 in the center of r1c1. A short list in one cell is what center marks are for.
- Keep both kinds of mark as the solve goes on. If r1c2 turns out to be 7, the corner 4 left in r1c3 becomes a placed 4, and the 7 comes off the center list in r1c1.
Practice
2 of 2
You know this cell can only be 3, 5 or 8. Which kind of mark records that?
Reference table
| Question | Corner marks | Center marks |
|---|---|---|
| What they record | Where one digit can go in a box, row or column | Which digits one cell can be |
| Best time to use | A digit has 2 or 3 spots in a box | A cell has 4 or fewer options |
| Patterns they show | Pointing, hidden singles, position pairs | Naked pairs and triples |
| What clears them | The digit lands in one of the marked cells | A digit lands in the cell's row, column or box |
| Common use in variants | A cage or line set whose order is unknown | The options a line rule allows in each cell |
Going deeper
How Snyder notation works
Snyder notation is a rule about when to corner-mark. Go through each digit and each box, and mark a digit only when it has exactly two possible cells in that box. Many solvers also mark three cells when those three sit in one row or column.
The limit keeps the grid readable. Two corner marks in a box make a pair of positions, and a digit placed in one of those cells settles the other. Marks lined up in one row or column show pointing without any extra work.
Do a full Snyder pass early, before the grid gets busy. After each new digit, check only the boxes that digit touches and add any new two-cell marks.
When center marks pay off
Center marks work best once a cell has four or fewer options. Writing all nine digits in every cell buries the useful cells among the noisy ones.
In variant puzzles, center marks are often the fastest way to record what a rule allows. A killer cage of 17 in two cells gets 89 in both cells. A cell on a German whisper line gets 1234 or 6789 once you know which side of 5 it's on.
Two cells in one house with the same two center marks are a naked pair. Three cells whose center marks use only three digits are a naked triple, even when no single cell shows all three.
Colors and mixed marks
Many apps also let you color cells. Color helps when two cells must be the same digit or must differ, and you don't know the digit yet. Give one cell green and its partner orange, then follow the chain until one color is placed.
Some solvers put both kinds of mark in one cell, with a corner digit for position and a short center list. That works as long as you stay consistent. Corner marks mean where a digit goes. Center marks mean what a cell holds.
Common mistakes
Marking every candidate
Filling cells with corner marks for every possible digit hides the patterns you're looking for. Mark a digit in a box only when it has two spots, or three in a line.
Mixing up the two meanings
A corner 4 means the box's 4 is in one of the marked cells. A center 4 means this one cell might be 4. Reading one as the other leads to false singles.
Leaving stale marks
When you place a digit, clear it from the marks in its row, column and box. A leftover mark can hide a single or make a pair look like a triple.
In mixed-rule puzzles
Each variant rule changes what you mark. Killer cages and sandwich clues often fix a set of digits before you know the order, which suits corner marks across the cage or center marks in each cell. Line rules such as whispers, kropki dots and thermometers limit each cell's options, so center marks fit them best. In a puzzle with several rules, write the result of each rule as marks right away. The useful deductions come when marks from two rules meet in the same cell.
Questions solvers ask
What are pencil marks in sudoku?
They are small notes in a cell that record possible digits before you place one. Most sudoku apps offer corner marks and center marks.
What is the difference between corner and center marks?
Corner marks show where a digit can go in a box, row or column. Center marks list the digits a single cell can be.
What is Snyder notation in sudoku?
It is a corner-marking habit where you note a digit in a box only when it has two possible cells there. Some solvers also mark digits with three possible cells.
How do I enter pencil marks on Variant Sudoku?
Hold Shift and type a digit for a corner mark. Hold Ctrl (Cmd on a Mac) for a center mark, and hold both for colors.
Should I use corner or center marks in sudoku?
Use both. Corner marks record where a digit can go in a box. Center marks record what a single cell can be. Most solvers start with corner marks and add center marks once a cell has a short list.
Is it cheating to use pencil marks in sudoku?
No. Notes only record logic you've already found. Hard puzzles, and most variants, expect you to take notes.
How many pencil marks should a cell have?
Aim for four or fewer center marks in a cell. For corner marks, mark a digit in a box when it has two possible cells, or three in one row or column.