Polyiamonds: A Spotter's Guide

Introduction

Polyiamonds are the shapes made by connecting equilateral triangles edge-to-edge. What sort of polyiamond you've got depends on how many triangles it contains. If it's just the one triangle, you've got what could be called a moniamond or mono-iamond, but is probably better off just being called an equilateral triangle, for everyone's sake. Two equilateral triangles stuck edge to edge creates a rhombus, or diamond if we're using the 'iamond suffix.[1]

Now stick a third triangle onto one of the sides of this diamond. This creates the legendary triamond, which is just half of a regular hexagon. Now when it comes to adding a fourth triangle, things get interesting. Interesting enough to warrant a diagram.

Fig. 1: Can't beat a bit of MS Paint, eh?

There are three unique ways of attaching that fourth triangle, creating the three tetriamonds. There's not as much variation here as there is with the square-analogues, the tetrominoes. In fact, as you increase the number of triangles the total number of polyiamonds increases, but at a much slower rate than the polyominoes.

Fig. 2: Numbers of free n-ominoes, n-iamonds and n-hexes for n=1, ..., 10. Polyhexes grow stupid fast, but that's a story for another time.

This of course is all assuming we're talking about 'free' polyiamonds anyway. That's free as in they can be rotated and flipped over, not free as in there's a bunch of unlockable shapes as in-game purchases or anything. If we allow the pieces to be rotated but not turned over, we get the sets of 'one-sided' polyiamonds, and if we disallow rotation too we get the 'fixed' polyiamonds. If we disallow rotation but allow pieces to be flipped over it would just be weird.

The 3 free, 4 one-sided and 14 fixed tetriamonds.

Pentiamonds

There are four free pentiamonds. And that's just not a big enough set to do a great deal with. But by considering reflections as distinct (i.e. the one-sided pentiamonds) we get a set of six which cover a total area of 30 triangles. This allows the set to be packed into a selection of little shapes, including a 3x5 parallelogram and these wobbly approximations of rectangles. And a dumpy not-quite-regular hexagon.

My recommendation at this point is to hack a set of these out of the back of a cereal box or something, they don't have to be exact as long as you can tell which piece is which, and have a crack at finding alternate solutions for the above shapes, or trying to find your own. It's also the kind of thing that could keep a child (or simple-minded adult, I raise my hand sheepishly) occupied for a while... Whole minutes of action-packed fun.

Hexiamonds

Now we're getting to the good stuff. Grab one of your pentiamonds from before, slap another triangle on there and you've got yourself a hexiamond. Keep doing it and eventually you'll wind up with the full set of 12 different shapes, each more spiky and irregular than the last. Twelve - just like the pentominoes. And if you're familiar with the pentominoes (and let's face it, if you aren't what are you doing on a site called Polyominoes The Blog anyway?) You'll know that you can do an awful lot with those twelve pieces. So this beckons the question, are the hexaimonds as versatile, as co-operative, as fun as their square siblings?

Well, the total area the hexiamonds cover is 6x12 = 72 unit triangles, which is promising. 72 divides up in various ways which suggests a selection of parallelograms at the very least - 2x18, 3x12, 4x9 and 6x6.

Fun fact: centre-aligned parallelograms always look off-centred. Though to be honest I didn't even try in this instance. Just slapped them into Paint and thought 'good enough'.

2x12 doesn't go. Too skinny. 3x12 doesn't either although you can get fairly close - real close in fact, 11 pieces - but the last piece just doesn't fit.

Close, but no cigar. (If the piece left over looked a bit like a cigar then that would have been way cooler.)

4x9 and 6x6 though, these are most definitely doable. There are 74 and 156 ways of solving these respectively (not counting rotations and reflections of the same solution) but they are deceptively tricky. That cereal box you cut your set of one-sided pentiamonds out of? Go find it, dig it out of the bin if necessary, turn it over and make a set of these pieces too. Yeah, you might have to turn the pieces over to find solutions with them and one of them might have 'RN FLAKES' or something written on one side. Too bad.
Alternatively, the board game 'Blokus Trigon' has pieces which are the stes of 1 through 6-iamonds. So if you have that you could use those. You even get a nice little board that allows the pieces to click in place when you're solving. This is what I did until I discovered the wide world of laser cutting (and the wide world of having the money to afford to get stuff custom laser cut.)

Any excuse to show off this set I made, complete with a nice little tray to hold them.

And if you get sick of parallelograms, there's no shortage of other shapes to solve too. There's the above four-row-high stretched hexagon for starters (purposely shown half-solved to entice readers to try and finish it themselves) and loads more - as long as its total area is 72 triangles and it's not too weird you're good to go.

Folks more artistic than me will be able to find all manner of solutions, I just tend to go for the obvious geometric shapes even though the symmetry of the base triangle lends itself well to snowflake-like sixfold symmetry. Think of them like a more frustrating version of Tangrams.

Heptiamonds

A deluxe wooden set of heptiamonds ('Deluxe' even though because wood was cheaper than acrylic...)

There are 24 heptiamonds (from here on in we're just going to concentrate on the 'free' ones) and at this point things get a little bit tougher. The pieces are just that little bit wigglier around the edges and that makes putting them together all the more frustrating. Various parallelograms can be made (see the image above), as well as a triangle with edge length 13 and one little triangular hole in the very centre. There's a bunch of other solution shape ideas over at the Polyform Puzzler site too, if looking at heptiamond solutions is your sort of thing.

One of the few shapes I've actually managed to solve by hand with the heptiamonds.

One of the (many) things that makes this set so tricky to solve with is the fact that there's relatively few pieces that fit nicely in a 60° corner. And when you're making a shape with triangles as a base unit, there's usually a few 60° angles knocking about.
I've noticed that when I solve by hand with these I tend to want to keep the longer thinner smoother pieces for the end. Whether that's any help whatsoever is up for debate, mind you. In fact that might explain why I'm so bad at solving these.

Another handy tip of course is just to get some polyiamond solving software going (Peter Esser's trisolve is the one that immediately springs to mind) - these make mincemeat of nearly every outline shape you can throw at it. Mincemeat in the good way.

Octiamonds

Now we're reaching the serious business. There are 66 of them which gives a total area of 528 triangles. And at this point the most popular technique for large polyomino constructions can be applied. That is, the age-old art of saving the more chunky blocky straight-edged pieces for the end. There's not a lot of octiamonds that fit this description but there's a few; enough to make the task of solving by hand a little less daunting than it first seems.

These guys. These absolute legends.

Sets of octiamonds are available from Kadon Enterprises. It's where I got mine, and it can be where you get yours if you're that way inclined. If not, lots of cereal boxes. You know the drill.

Enneiamonds

I don't know why the 'non-' prefix wasn't good enough for this lot but clearly it isn't. Just try googling 'noniamonds' and marvel at the complete lack of results that show up. 'Enneiamonds' on the other hand turns up a few sites (maybe even this blog if it gets popular!) with constructions from polyformists (is that a word?) far more dedicated than I.

An interesting thing happens when you reach the enneiamonds too. It's this:

Nine triangles can be made to completely surround another triangle, allowing for the fabled holey enneiamond to exist. Not quite as infamous as its distant relative, the harbour heptomino, but getting there. This makes it a little harder to fill shapes but it can be worked around, usually by introducing a few more holes and placing them symmetrically throughout the construction.

I've got a set of enneiamonds, laser cut out of four colours of acrylic in a way that makes them look very like primary school teaching aids, but as of yet I haven't finished any constructions with them. Peeling the protective sticky material off both sides of each piece was an ordeal enough.

Deciamonds or Dekiamonds, I'm not sure 

There are 448 of these, 4 of which have holes in them. Nevertheless, some brave and intrepid individuals have made things with the full set. Check out this over the the Poly Pages by Patrick Hamlyn. And this page which has a solution by John Greening, using a combination of several sets, all one inside the other like Russian dolls or something.

The 6- through 10-iamonds, found by John Greening (more information at the above Kadon Enterprises link.) I drew this up based on the photo on that page, but the photo itself is unclear in some places so I may have made mistakes.

Beyond these though, you're pretty much stepping into the unknown, venturing into unexplored territory.
Or if it's not unknown, there's certainly not enough pretty pictures of it up on the internet.

---

[1] FUN FACT: This isn't the etymology of the word diamond. The word 'diamond' came first and the suffix '-iamond' was back-formed from it at a later date.