The Rule

Every Trigon is built from small triangles. Each side carries a number, and the number in a triangle's centre is always the sum of its three sides. Where two triangles meet, the number on that shared side is the same for both — work it out by logic and elimination against the key, the same way you would on paper.

Never put in a number because it will fit. Only put in a number because nothing else will.

Playing On Screen

This app is designed to work on a phone, tablet, laptop or desktop. The smaller screens should be able to see all of the smaller puzzles and key areas, but on the larger puzzles you can zoom and pan on the area you are working on.

In the printed book, you write each side's number in by hand. Here you select a triangle, and it turns yellow; it also shows that triangle's centre number in the row of triplets in the key. Click on the triplet you want and a floating triangle appears. You can invert it by clicking the top point, then drag the numbers from one side to the other to swap places. When you are happy, drag it over the puzzle to where you want to drop it. As it passes over other triangles, it shows that triangle's centre number, so make sure you are dropping it in the right place.

The squares in which the number sits are red. This shows it is pencilled, i.e. you are not sure it is a fit; you may have several of these while you try out options. When you are sure, to quote Dax:

“Never put in a number because it will fit. Only put in a number because nothing else will fit.”

Then tap Ink to lock it in. The squares turn blue. They are fixed unless you press Oops, which undoes your last action. Prefer one box at a time? Tap a number badge at the left of the Key, then tap any box that isn't already inked to write just that digit. Oops undoes your last move, one step at a time; Check tells you how many boxes are filled and how many are right so far, any time you want to know.

Back / Forward step through this Mini Book's puzzles in the same order as the printed pages. Save / Load keep every puzzle's progress separate, so you can put one down and pick up exactly where you left off. Fit / Centre and the board's own +/− zoom let you get in close on a crowded corner without losing your place; the Key panel zooms independently the same way.

Settings also holds Reveal (reference) (shows the real solved shape at a glance, without touching anything you've filled in) and Save My View / Go to My View (park your zoom and pan exactly where you like it, and jump straight back any time). On a phone, tap the ▾ Menu / Settings strip at the top to tuck the buttons away and give the board more room — tap it again to bring them back.

Help gives you a real next step: the forced move if there is one, or the safest guess if there isn't, worked out live from what's actually on the board right now — not a canned tip. It's switched on for this Mini Book's 3- and 4-series puzzles for now; the bigger 5- and 6-series boards are still being tuned for it.

Menu (this button) brings this screen back any time.