A nonogram is a picture-logic puzzle solved with nothing but numbers. You will also see it called picross or a griddler โ those are the same puzzle under different names from different publishers, and it helps to know that when you're searching for more of them. Every row and every column carries a clue: a short string of numbers telling you how many filled squares sit in that line and how they're grouped. Fill the right squares, leave the rest empty, and a hidden picture appears. You can play a full 20x20 board right now at Nonogram, switching between Pencil mode to paint a square solid and X mode to mark one you're sure is empty, then click-and-dragging across cells to apply whichever mark you need.
This guide covers the techniques that solve nonograms without guessing: reading a clue correctly, the overlap trick that starts almost any line, why marking empty cells matters as much as filling them, edge forcing, splitting a line at a finished block, recounting free space, and working across rows and columns instead of grinding one line.
How to Read the Clue Numbers
A clue like "4 2 1" on a row means: reading left to right, a block of 4 filled squares, then at least one empty square, then a block of 2, then at least one empty square, then a block of 1 โ nothing else filled in that row. Column clues read the same way, top to bottom. Two things trip up beginners: the order of the numbers is fixed to the direction of the line, and blocks are never adjacent โ there is always at least one empty cell separating two numbers in the same clue, even if the puzzle doesn't draw that gap. A single number like "7" simply means one run of seven filled cells somewhere in the line, everything else empty.
Overlap: The Technique That Gets You Started
Overlap is the single most useful beginner technique, because it works on almost any line the moment you look at it, with no other information required. When a clue number is large relative to the length of the line, the block has very little room to slide, so some cells in the middle must be filled no matter where it actually sits.
Here's a worked example on a line 10 cells wide with a single clue of "7". Push the block of 7 as far left as it will go: it fills cells 1 through 7, leaving cells 8, 9, and 10 empty. Now push it as far right as it will go: it fills cells 4 through 10, leaving cells 1, 2, and 3 empty. Compare the two placements. Cells 4, 5, 6, and 7 are filled in both the leftmost and the rightmost version of the block. Since the real block has to sit somewhere between those two extremes, it always covers cells 4 through 7 โ so you can fill those four cells immediately, before you know anything else about the line. The general rule: subtract the line length from the clue number, and that difference tells you how much "slack" the block has to slide; whatever's left over in the middle after accounting for that slack is forced.
Marking Empty Cells Is Just as Valuable as Filling Them
New players tend to only use Pencil mode and ignore X mode until they're stuck, but marking a cell as definitely empty is exactly as much progress as filling one in โ both remove uncertainty from the board. If a row's clue is a single "3" and you've worked out from the columns that one cell can't be part of any block, marking it with an X shrinks the space the block of 3 can occupy, which can trigger a fresh overlap calculation or fully determine the line. Mark every cell you can rule out, not just the ones you can confirm โ eliminating possibilities often unlocks the next deduction.
Edge Forcing
Sometimes a block is forced flush against the edge of the line, not because of overlap, but because there isn't enough room anywhere else for every number in the clue to fit. Take a line 8 cells long with the clue "6 1". The minimum space this clue can occupy is 6 filled cells, plus 1 mandatory gap, plus 1 more filled cell โ exactly 8, the entire line. When a clue's minimum required space equals the line length exactly, there's no slack at all, and the whole line is fully determined on the spot: cells 1 through 6 filled, cell 7 empty, cell 8 filled. Even with a little slack, the block nearest an edge is often still forced to start right at that edge, because starting it even one cell later would leave too little room for the remaining numbers to fit with their required gaps.
Splitting a Line at a Completed Block
Once a run of filled cells is bounded on both sides by confirmed empty cells (or a grid edge) and its length matches one of the clue numbers, treat it as finished and stop thinking about it. That block splits the line into two independent shorter puzzles: everything to its left satisfies the numbers before it, everything to its right satisfies the numbers after. On long lines with several clue numbers, this turns one hard problem into two much smaller ones that don't interact.
Counting the Remaining Free Space
Every time you fill or eliminate cells in a line, recalculate how much free space is left for the clue numbers not yet placed. A clue with plenty of slack early on can become tightly constrained once neighboring blocks are confirmed โ and a tightly constrained clue is exactly where overlap and edge forcing become useful again. Revisit a line more than once as the rest of the grid fills in, rather than treating a first pass as final.
Alternate Between Rows and Columns
Don't grind a single row until it's fully solved before touching anything else. Fill in what a row's clue forces, then immediately check the columns those cells belong to โ a single filled or empty cell can unlock an overlap or an edge force in a column you hadn't touched yet. Short passes across both directions spread information through the puzzle far faster than exhausting one line at a time, and it's usually the difference between steady progress and getting stuck.
Trust the Logic โ Never Guess
A properly designed nonogram always has exactly one solution reachable through logic alone; you never need to guess. That matters because a wrong guess doesn't just cost one cell โ it can look locally consistent for several more moves while you build on it, and by the time a contradiction shows up elsewhere on the grid, you may have corrupted a large section of your work with no clean way to tell which fill caused it. If nothing seems forced, stop and recheck a different row or column instead of placing a cell on a hunch; the deduction is always there, even when it isn't obvious yet.
Common Mistakes
- Guessing when stuck instead of switching lines. Move to a different row or column and look for a fresh overlap rather than filling a cell you're not certain of.
- Only using Pencil mode. X mode is not a bookkeeping afterthought โ marking empty cells drives just as many deductions as filling solid ones.
- Treating clue numbers as if blocks can touch. Two numbers in the same clue always have at least one empty cell between their blocks; forgetting that leads to miscounted overlaps.
- Solving one line to completion before moving on. This wastes the information a partially solved row could have handed to its columns much sooner.
- Not recalculating free space after neighboring lines change. A clue that looked unsolvable at a glance often becomes forced once you subtract newly confirmed empty cells.
Frequently Asked Questions
What's the difference between a nonogram, picross, and a griddler?
Nothing โ they're the same puzzle under different regional names. Nonogram is the generic term used across most puzzle sites, including this one.
Do the numbers in a clue have to stay in order?
Yes. The sequence always matches the order the blocks appear in the line, left to right for rows and top to bottom for columns.
Is guessing ever necessary?
No. Every square in a properly built nonogram can be determined through deduction, so guessing is a risk you never need to take.
What does the overlap technique actually prove?
It proves which cells are filled in every valid placement of a block, so those cells must be filled regardless of where the block actually sits.
How big is the grid on this site?
The nonogram at Nonogram is a 20x20 grid, with a hidden picture that gradually appears as you fill in correct cells.
What should I do when I can't find any forced cell?
Switch to a different row or column and recount its free space rather than guessing โ a deduction elsewhere often unlocks the line you're stuck on.
Related Guides
- How to Play Minesweeper โ another pure-logic grid puzzle where marking what you're certain about is just as important as revealing new cells.
- Sudoku Strategy Guide โ for more elimination-based deduction techniques that transfer directly to nonogram thinking.
- How to Solve Sokoban Puzzles โ a completely different kind of logic puzzle: spatial planning instead of grid deduction.
- How to Play Mahjong Solitaire โ a calmer tile-matching puzzle if you want a break from number logic.
- A Beginner's Guide to Puzzle Games โ a wider starting point if you're new to logic puzzles in general.