A group on a 9x9 block board is a row, a column or a 3x3 box, and it clears when all nine of its cells are filled. A piece completes a group only if it covers every empty cell that group has left. So a double clear, two groups at once, comes down to one condition: the empty cells of both groups must fit inside a single piece. That condition can be counted before you commit to anything.
The counting rule
Call the empty cells of a group its gaps. For two groups, the piece must cover every gap in either, and a gap that belongs to both is only covered once:
piece cells = gaps in the first group + gaps in the second − gaps they share
Two groups of the same kind, such as two rows, never share a cell, so nothing is subtracted. A row and a column share exactly one cell, where they cross. A row and a box it passes through share up to three. The earlier post on the 9x9 grid has the full overlap counts.
The diagram shows the commonest case. A row and a column each have two gaps, and the crossing cell is one of them, so the piece needs 2 + 2 − 1 = 3 cells, and a corner-shaped piece of three fits exactly.
Six set-ups, counted
The same arithmetic gives the cells a double clear frees. Cleared cells are the union of the groups: 9 + 9 less any overlap. Subtract the piece's cells and what remains must already be filled.
| Set-up | Piece cells | Cells cleared | Already filled |
|---|---|---|---|
| Two adjacent rows, 1 gap each, same column | 2 | 18 | 16 |
| Two adjacent rows, 2 gaps each, same two adjacent columns | 4 | 18 | 14 |
| Row and column, 2 gaps each, crossing at a gap | 3 | 17 | 14 |
| Row and column, 3 gaps each, crossing at a gap | 5 | 17 | 12 |
| Row and box, 3 gaps, all in the box | 3 | 15 | 12 |
| Row, column and box, 2 + 2 gaps meeting in the box | 3 | 21 | 18 |
Each line can be checked by hand. Two adjacent rows are 18 cells; one gap each means an upright domino, so 16 are filled, and two gaps each means a 2x2 square, so 14 are filled. A row and a column cover 9 + 9 − 1 = 17 cells. A row and a box that share three cells cover 9 + 9 − 3 = 15.
The last line is the triple. Put the L-shaped piece in the corner of a box so that its row gaps and column gaps all lie inside that box, with the box's other six cells full. The row, the column and the box all complete, and the union is 21 cells, a little over a quarter of the board.
The larger the piece you are waiting for, the less of the board you need to have filled. A domino double clear needs 16 cells in place, a 2x2 needs 14, and a plus-shaped piece across a row and column needs only 12.
Why the shape matters more than the count
The table has a catch: more cells in the piece means fewer to fill beforehand, but it also means fewer pieces will fit. A domino slot is the easiest to build, because many shapes can put a vertical pair of cells there. A plus-shaped slot needs a plus, and gaps that wait for one shape are cells you cannot use meanwhile.
Two limits follow from the rule.
- The gaps must be connected. A piece is made of cells joined edge to edge, so the combined gaps have to touch edge to edge. Two rows each missing cells in columns 2 and 5, say, cannot be completed together, because no single piece spans the filled cells between them.
- The crossing cell must be a gap. If a row and a column cross on a cell that is already filled, the row's gaps and the column's gaps touch each other at most at a corner, so no connected piece covers both. The L in the diagram works only because the crossing cell is empty.
Big Ben Blocks: Block Puzzle uses the 9x9 grid with rows, columns and 3x3 boxes, and the Adventure levels set line-clearing goals. Each level has a fixed starting board and piece sequence, so a set-up that works can be worked out once and repeated.
Building the second line in step
The practical habit is to build two neighbouring lines together rather than one at a time. Fill both rows left to right and stop at the same column, and you leave a slot two cells deep that any piece able to put two cells one above the other will fill.
Two rules of thumb follow from the counts:
- Keep gaps aligned. Two rows with gaps in the same columns can be cleared together. Gaps that are staggered need two separate pieces.
- Look at the box too. If a row's gaps and a column's gaps both lie in a box that is otherwise full, the same piece clears all three, for 21 cells.
When not to wait
Of the double clears above, two parallel lines free the most, 18 cells, against 9 for a single line. That is only twice the room, and a slot held open costs cells until the right piece comes. If the board is filling up, a single clear taken now beats a double that needs a shape you may not get.
The untimed Classic mode is the easiest place to practise the counting, since there is no clock pressing you to place a piece before you have checked the gaps.
Big Ben Blocks: Block Puzzle offers untimed Classic play or a three-minute Time Attack, and the Daily Challenge changes at midnight UK time. The ten Adventure levels have line-clearing and combo goals and a fixed starting board and piece sequence. Blocks carry shape symbols as well as colours. It plays offline with no ads and no in-app purchases, and the coins you earn have no cash value.