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Tap every digit that could go in the middle of the shaded region.
Select all that apply
Level 5 · Outside clues and specials
How to solve extra regions sudoku
A shaded region works like an extra box: no digit repeats inside it.
Extra regions sudoku shades groups of cells in the grid. Each shaded region follows the box rule, so no digit can repeat inside it. A nine-cell region holds 1 to 9 exactly once.
Extra regions give you another group to scan for singles. Because they cut across rows, columns and boxes, they link parts of the grid that the normal rules keep apart.
How setters write it
The same rule, worded the ways you’ll see it on puzzles.
- Digits may not repeat within a shaded region.
- Extra regions.
- Each gray region contains no repeated digits.
Worth memorizing
- A 9-cell extra region holds 1 to 9 exactly once, just like a box.
- Regions that cross boxes create new places for singles to appear.
Solving tips
Scan regions for singles
Treat a nine-cell region like a box. If it has eight digits placed, the ninth cell gets the missing one.
Use the overlap with boxes
If a region shares three cells with a box and the box's 7 must go in those shared cells, the region's 7 is there too. Remove 7 from the region's other cells.
Know the window layout
One layout shades four 3x3 windows, each set one row and one column in from a corner of the grid. Each window holds 1 to 9 once, just like a box.
Placed digits reach across boxes
A digit in a region rules it out of every other cell in that region, even across box borders. A 5 in one corner of a region removes 5 from region cells in a different box.
A worked example: a hidden region in hyper sudoku
This is a hyper sudoku. Four shaded 3x3 windows cover rows 2 to 4 and rows 6 to 8, in columns 2 to 4 and columns 6 to 8. We name cells by row and column, so row 5, column 3 is r5c3. Look at the nine cells in rows 1, 5 and 9 of columns 2, 3 and 4. Eight of them hold 1, 2, 3, 4, 5, 6, 8 and 9. Only r5c3 is empty.
- Columns 2, 3 and 4 hold 27 cells. Each column has 1 to 9 once, so each digit appears exactly three times in those three columns.
- The top-left window (rows 2 to 4) and the bottom-left window (rows 6 to 8) both sit inside those columns. Each window holds 1 to 9 once. Together they use two copies of every digit.
- The cells left over are rows 1, 5 and 9 of columns 2 to 4. They must hold the third copy of every digit. So those nine cells hold 1 to 9 once, just like a box.
- Eight of those cells show every digit except 7. So r5c3 = 7, even if its row, column and box still have other gaps.
Practice
Reference table
| Region | Rows | Columns |
|---|---|---|
| Hidden region 1 | 1, 5, 9 | 2 to 4 |
| Hidden region 2 | 1, 5, 9 | 6 to 8 |
| Hidden region 3 | 2 to 4 | 1, 5, 9 |
| Hidden region 4 | 6 to 8 | 1, 5, 9 |
| Hidden region 5 | 1, 5, 9 | 1, 5, 9 |
Going deeper
Finding hidden regions by counting
The worked example uses a counting trick. Three columns hold three copies of each digit. If shaded regions inside those columns account for two copies, the cells left over hold the third. The same trick works on three rows.
The table lists the five hidden regions this gives in hyper sudoku. Treat each one like a box. Scan it for singles, and use its overlaps with boxes, rows and columns.
You can use the same trick on any puzzle with nine-cell extra regions. Pick three rows or three columns. Take away the regions that sit fully inside them. If exactly nine cells are left, they hold 1 to 9 once.
Regions with fewer than nine cells
A region with fewer than nine cells only needs different digits. A four-cell region could hold 1, 2, 3 and 4, or 2, 5, 7 and 9. It doesn't have to hold any digit in particular.
Small regions help most when they cross box borders. Two cells in different rows, columns and boxes could match under normal rules. If they are in the same region, they can't.
If a three-cell region sits in three different rows, columns and boxes, no normal rule links its cells. The region is the only thing that stops them from matching.
Regions that cross box borders
When a region shares cells with a box, look at one digit at a time. If the region's copy of a digit can only go in the shared cells, the box's copy is there too. Remove the digit from the rest of the box.
The reverse also works. If the box's copy of a digit must go in the shared cells, remove it from the rest of the region. This is the same overlap logic you use between rows and boxes. For a region that crosses three boxes, check each overlap on its own.
Common mistakes
Expecting 1 to 9 in a small region
Only a region with nine cells must hold 1 to 9. A smaller region just can't repeat a digit. Don't assume a four-cell region holds 1 to 4.
Mixing up shading from other rules
Gray shading can mean an extra region, an odd or even cell, or something else. Read the rules to see what the shading means in that puzzle.
Ignoring the regions after the start
Beginners use the shaded regions early and then forget them. They keep giving singles all the way to the end. Scan each region along with the rows, columns and boxes every time you are stuck. In hyper sudoku, add the hidden regions to that scan.
In mixed-rule puzzles
Extra regions add more groups to other rules. With diagonals, the long diagonals cross the hyper windows, so you get more overlaps to scan. With killer cages, a nine-cell region adds up to 45, since 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45. That lets you work out missing cage totals the same way you would for a box. With anti-knight or anti-king, each placed digit rules out cells in its region as well as its row, column and box. With quads, a quad inside one region can't repeat a digit. Each puzzle's rules say which shading counts as a region, so check them first. With odd-even marks, a nine-cell region needs five odd digits and four even ones, like a box.
What the video shows
- A shaded region works like an extra box: no repeated digits.
- Two 5s in the same shaded region.
- 5, 1 and 8 are all different.
Questions solvers ask
What is extra regions sudoku?
A sudoku with shaded groups of cells that follow the box rule. Digits can't repeat inside a shaded region.
What is hyper sudoku?
Hyper sudoku, also called windoku, adds four shaded 3x3 regions to the grid. Each of them must hold 1 to 9 once.
Do extra regions always have nine cells?
No. A region with nine cells holds 1 to 9 once, and a smaller region only needs its digits to be different.
Can extra regions overlap boxes?
Yes. A shaded region can cross box borders, and the box rule and the region rule both apply to the shared cells.
Are there hidden regions in hyper sudoku?
Yes. Five sets of nine cells must hold 1 to 9 once even though nothing marks them. The table on this page lists them, and the worked example shows why.
What does gray shading mean in sudoku?
It depends on the puzzle. It can mark an extra region, an odd or even cell, or another rule. The rules list tells you which.
Do extra regions add up to 45?
A region with nine cells holds 1 to 9 once, so its digits add up to 45. A smaller region has no fixed total.
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