Chute Scanning in Sudoku: How Bands and Stacks Reveal Placements

Understand bands, stacks, and chutes

The Sudoku grid contains three horizontal bands, each made of three boxes, and three vertical stacks, also made of three boxes. Solvers sometimes use the word chute for either kind of three-box strip. Every symbol must appear once in each row and column, so placements inside one box restrict matching positions in the other two boxes of the same chute. Suppose two hearts are already placed in different rows of the first two boxes in the top band. Those rows are blocked for the heart in the third box, which may leave only one row available there. You can then narrow the heart to one or more cells within that row. This is broader than inspecting one box in isolation. The method uses the repeated three-box structure to carry information sideways or vertically. In Shapedoku, following one glowing shape through a full chute can be easier than jumping between unrelated empty cells.

Scan mini-lines inside each box

Each box contains three short horizontal mini-rows and three short vertical mini-columns. When scanning a horizontal band, compare the three mini-rows that line up across its boxes. If a candidate is confined to one mini-row in the first box and another mini-row in the second, the remaining box may be forced into the third mini-row. The same idea works vertically through stacks. This does not always solve a cell immediately, but it can create a pointing pair, a claiming pattern, or a hidden single. Chute scanning is especially effective when the grid has enough placed symbols to block whole mini-lines but still has too many open cells for ordinary singles. Work with one shape at a time and complete the whole band or stack before switching symbols. A consistent order keeps the scan complete and prevents useful restrictions from being forgotten.

Turn chute restrictions into exact deductions

After identifying the only possible mini-line for a shape in one box, combine that restriction with the crossing columns or rows. If the heart must lie in the bottom mini-row of a box, check each cell in that mini-row against its column. Two columns may already contain hearts, leaving the third cell forced. If two cells remain, mark them in Notes and use the pair as a box-line restriction. The shape cannot appear elsewhere on that row inside the box, and later placements may collapse the pair. Chute scanning works well on Medium and Hard puzzles because it strengthens basic crosshatching without requiring a named advanced pattern. On Extreme boards, it is still worth performing before chains or fish, since a missed chute restriction can make a difficult puzzle look far more complex than it is. Repeat the scan after every placement that completes or nearly completes a box.

1. Choose one shape and one horizontal band or vertical stack.
2. Mark which rows or columns are blocked by existing placements.
3. Compare the three aligned mini-lines across the chute.
4. Identify the remaining mini-line available in the unsolved box.
5. Use crossing units to reduce that mini-line to one or two cells.
6. Repeat through all chutes before moving to advanced techniques.