Hard Between Lines Sudoku

Digits on each line must fall between the values at its endpoints. Every line adds a range constraint to the standard rules.

Easy Medium Hard Expert

Hard Between Lines Sudoku. Long lines with wide ranges require iterative endpoint narrowing — track range bounds carefully as you place digits across the board.

2
3
1
9
6
2
4
1
6
8
4
1
3
7
1
6
4
7
8
3
7
2
9
8
4
6
2
Mistakes
0/3
Score
-
Time
00:00
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Progress0%

What is Between Lines Sudoku?

Difficulty
★★★☆☆
3/5
Constraint Type
Line Constraints
Typical Givens
20–28
Avg. Solve (Hard)
36 min

Solving Techniques for Hard Level

Technique Description Level
Endpoint Gap Analysis The gap between endpoint values must be large enough to accommodate all middle cells. If endpoints are 2 and 8, middle cells must come from {3,4,5,6,7}. Beginner
Endpoint Ordering The endpoint values have no fixed order — the constraint is that all middle cells are strictly between the minimum and maximum endpoint. Beginner
Tight Endpoint Constraint If endpoints would be consecutive (e.g., 3 and 4), no middle cell could satisfy the strict inequalities — use this to force endpoints apart. Intermediate
Between-Line–Box Interaction All middle cells sharing a box must satisfy the between constraint. Use the box to eliminate candidates outside the valid range. Advanced

Ready for the ultimate test? Try Expert Between Lines Sudoku — the hardest puzzles on Sudoku.by.

Techniques to Master at Hard

  • Hidden Pairs technique — Range arithmetic deletes middle candidates from line cells silently; recount the boxes a long line crosses, because hard between grids hide pairs behind out-of-date ranges.
  • X-Wing technique — The extremes 1 and 9 only live off-line, so their candidate maps are sparse from the start — hard between puzzles are usually cracked by an X-Wing on 1 or 9.

Average Solve Time by Difficulty

Easy
8 min
Medium
18 min
Hard
36 min
Expert
64 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Between Lines Sudoku →

Related Variants

Palindrome Sudoku

Another variant where a line links far-apart cells — mirrored digits there, a shared value range here.

German Whispers

Inverts the comfort zone: between lines squeeze digits into a narrow range while whispers force them far apart.

Thermo Sudoku

Adds full ordering to the range logic you use on between lines — every step along a thermometer must increase.

Frequently Asked Questions — Hard Between Lines Sudoku

What makes hard Between Lines Sudoku difficult?
Hard Between Lines puzzles have long lines and unknown endpoint values, making the valid range for middle cells very wide initially. You must iteratively tighten the endpoint ranges as you place digits, creating a feedback loop between standard Sudoku logic and between-line constraint propagation.
How do I narrow endpoint values in a hard Between Lines Sudoku?
Look at the middle cells of a line and find the most constrained cell — the one with the fewest candidates. The endpoints must be strictly outside the range of all valid middle candidates, so the more you restrict the middle cells, the tighter the bounds on the endpoints become.
What is a between-line contradiction test?
A contradiction test hypothetically assigns a value to one endpoint and checks whether valid middle-cell candidates still exist given that value. If assuming endpoint = 2 leaves no valid middle candidates (all remaining candidates are ≤ 2), then the endpoint cannot be 2. Repeating this test for each candidate quickly pins down the endpoint.
How do long between lines interact with standard Sudoku constraints?
Long lines pass through multiple rows, columns, and boxes. Each segment of the line generates between-constraint restrictions for the cells it touches. In hard puzzles, tracing how a single endpoint value propagates restrictions through all these crossed units is essential for making progress.
How long does a hard Between Lines Sudoku take?
Expect 35–60 minutes for hard puzzles. Solvers who systematically track endpoint-value ranges for each line — rather than treating the between constraint as secondary — solve significantly faster.

More questions? See the full Between Lines Sudoku guide.