How to Solve Windoku (Hyper Sudoku) on Paper with a Pen

Windoku — also called Hyper Sudoku — is the variant that turns your mental model of Sudoku from nine boxes into thirteen. Four extra grey 3x3 windows are drawn inside the grid, each floating at a corner, and every one of them must hold digits 1–9 exactly once. On screen the window outlines are faint but always present. On paper, if you trust the printed grey, you will miss them.

The pen solver's job is to make those four hidden houses legible before the candidate markup begins. When you do, the extra houses multiply the power of every pointing pair and every intersection elimination in the pen-and-paper Series Hub — and they roughly double the number of hiding places a digit can complete. This guide builds directly on the strategy patterns in the strategy index and the X-Wing technique guide.

The two-pen rule for windows: draw the four window borders in red the instant you set up the board. From then on, your red pen does double duty — it encodes the extra houses and it records the strikes that flow through them.

1. The Precise Rules

• Apply all classic rules: every row, column, and 3x3 box contains digits 1–9 exactly once.
• The four grey 3x3 windows — each centred one box from a grid corner — must each ALSO contain digits 1–9 exactly once.
• The grid therefore has 4 extra hidden houses on top of the standard 9 rows, 9 columns, and 9 boxes — 31 houses in total.
• A cell can belong to a classic box and a window simultaneously; such shared cells are constrained twice.
• The four window centres lie on the inner 3x3; the windows themselves wrap around the corners but never the outermost box row or column entirely.

The numeric consequence is immediate: a window that has collected eight distinct digits forces its ninth with unusual authority, and a digit confined to a single line within a window annihilates that digit along the rest of the line — a pointing pair with double the reach.

2. Making the Windows Visible on Paper

The grey shading in a printout is the enemy of a clean board — faded, hard to distinguish from cross-hatching, and easy to ignore. Before Pass 1, perform the window census:

• Trace all four window outlines in red. Use a distinct stroke so a window's 3x3 boundary reads at a glance, separate from the classic box grid.
• Flag shared cells. Where a window overlaps the corner of a classic box, mark those doubly-constrained cells — they are your highest-leverage targets.
• Imagine each window as a float. The window is not locked to the outer border; it drifts over the box grid. Trace carefully so the float's rows and columns stay aligned to the real grid lines.

3. Step-by-Step Workflow

4. The Windoku Toolkit

4.1 Window Pointing Pairs

When a digit is confined to a single row (or column) within a window, that row position is the digit's only legal home in the window — so the digit cannot appear anywhere else on that same row or column outside the window. Strike outward from the window boundary. Classic pointing pairs reach one box; a window pointing pair can reach across two boxes at once.

4.2 Box-Window Intersection Elimination

Any cell shared by a box and a window is constrained by two 1-9 houses simultaneously. If the classic box already proves a cell cannot hold 4, the window ledger shares that prohibition — and vice versa. Prune the intersection candidate with the stronger of the two constraints, and when a window placement feeds back into the box, the doubled cell collapses fast.

4.3 The Window-Corner Trick

A window corner cell sits inside two overlapping houses beyond its own window. When a digit in a window corner is resolved, it strips that digit from the two overlapping houses' shared region at once — a single placement doing the work of two classic placements. Hunt window-corner cells first; they are the cheapest wins on the board.

4.4 Treat Windows as Pink-Slip Ledgers

Keep a running tally of each window's collected digits. A window with six distinct values is a near-certain source of singles; a window with eight is a forced ninth. Read the window as a ledger and the board as a system of 31 intersecting ledgers.

5. The Four Windows and a Pointing Pair

The grid below outlines all four windows with distinct tones and highlights a single window pointing-pair elimination for digit 3 in the upper window.

Read the elimination this way: the digit 3 in the upper window (highlighted) is the only 3 in its window row. Because the window must contain a 3 and it is confined to that one cell, no other 3 can live on that same row outside the window — hence the red strike down the row to the right. One window pointing pair has pruned outside the window entirely, across classic boxes.

6. Common Windoku Mistakes

• Skipping the window trace. Trusting faint grey is how a window single goes missing mid-solve. Trace the four outlines in red before any solving.
• Under-counting shared cells. A cell in a box and a window carries two 1-9 constraints. Treating it as a single-house cell leaves candidates alive that the second house kills.
• Excluding windows from the single scan. Hidden singles hide in windows just as often as boxes. Scan all 31 houses, not the classic nine.
• Forgetting the window corner reach. A resolved window-corner digit strips two overlapping houses at once — skipping it leaves a live candidate you could have killed cheaply.

7. Practice the Float of Houses

Start on Easy and Medium boards, tracing the windows first and hunting window singles before touching candidate markup. On Hard and Expert boards, the window pointing pair and window-corner trick become the difference-makers. Pair them with the reductions in the X-Wing technique guide and the strategy index.

Windows multiply houses; the final guide multiplies grids. When you can track four floating ledgers, you are ready for Samurai Sudoku's five-board gattai.