triangulo

Three-corner. Say it and the word falls open. Tri is three, at rank 174. Angulo is a corner or an angle, at rank 702. A triangle is a thing with three corners, and the Esperanto for it is exactly that, spelled out.

The pleasure here is that Latin did precisely the same thing two thousand years earlier - tres plus angulus gives triangulum - and English inherited the result as an opaque lump you have to learn. Esperanto did not inherit it. It rebuilt it, out of parts a beginner already holds, and arrived at the same word by the same road.

That means triangulo is a real compound, not a false seam like modifi or orkestro elsewhere in this band. The test is always the same: are both halves Esperanto words that combine to the meaning you have? Here they are.

The rest of the shapes, mostly for free.

• kvadrato - a square, and this one is a borrowed root
• rektangulo - a rectangle, right-corner
• kvarangulo - a quadrilateral, four-corner
• kvinangulo - a pentagon, five-corner
• rondo or cirklo - a circle
• plurangulo - a polygon, many-corner, from plur-

Notice what happens once you have the -angulo pattern: you can name any polygon you like without ever meeting the word for it. English makes you learn pentagon, hexagon, heptagon and octagon from Greek, one at a time.

The adjective. Triangula means triangular, and it is doing ordinary Rule 3 work: triangula formo, a triangular shape. Triangulaĵo would be a triangular thing, if you ever need it.

And the musical one. The percussion triangle in an orkestro is also a triangulo, which is one of those rare cases where a metaphor survives the crossing between languages completely intact.