How to play
- Goal
- Place every number from 0 to one less than the chart size once on each header edge, matching every printed sum.
- How it ends
- Both header edges are filled and all printed sums match; there is no loss state.
- Record
- Each chart keeps your first solve time, and an uninterrupted first solve can claim the named fastest time.
- Each edge independently uses every number from 0 to one less than the chart size, once. A 4×4 chart uses 0, 1, 2 and 3 on each edge.
- A printed interior number is the sum of its row header and column header. Zero is a real clue and a real header number.
- The same number may appear once on each edge. Repeating it within one edge is forbidden.
- Unclued crossings add no condition. Fill both edges and match every printed sum.
Controls
| Mouse / touch | Choose a number, then tap a header. Tap the same number again to erase it. Interior sums cannot be edited. |
|---|---|
| Arrows + Enter / Space | Move header focus and apply the selected number |
| Number keys / Delete | Enter a number directly / erase the focused header |
| Z / Y / R | Undo / Redo / open Restart confirmation |
| Pan / P | Move a large chart without entering numbers |
| Whole chart | Open a read-only overview from the chart menu |
| Escape | Close a dialog or return to the atlas |
| Controller | D-pad move; A apply; X erase; Y Undo; shoulders cycle numbers; B back; Start settings |
Tips for Summandum
- Start with zero A sum of zero forces both of its headers to zero. Those zeros belong to separate edges.
- Use the edge set Once an edge contains every number except one, its remaining header is forced.
- Check bounds If a known header exceeds a printed sum, their other header would need to be negative.
Questions about this game
Does zero mean empty?
No. A dot indicates an empty header or an unclued crossing. The numeral 0 is a real number.
Can the two edges share a number?
Yes. Each edge has its own complete set. Duplicates within an edge are forbidden.
Are shelves difficulty levels?
They describe sizes: eight 4×4, twenty-four 5×5 and sixty-four 6×6 charts. No difficulty ranking is promised.
Does Restart reset the clock?
It clears entries while retaining the original first-solve clock. In Practice, the first finish stays unchanged. Undo restores cleared entries.
What happens on replay or reload?
Completed charts open in untimed Practice. Reloaded unfinished charts retain entries and Undo, and their time stays local.
Charts and the worked example are original. Entries and Undo save in this browser. The first clock starts when a chart is selected. Reloaded attempts and detected clock or browser-suspension discontinuities stay local. Unsafe or unavailable storage is reported in-game. This is an independent implementation of Summandum puzzle rules.
Watch how to play
About Summandum
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