Nomes Dos Polígonos De 1 A 20

Polygon names from 1 to 20: what you actually need to know

The names of polygons go further than most people realize, and honestly, the first few numbers are where things get weird. A 1-sided figure isn't really a polygon by any definition that matters in practice. Same with a 2-sided one. They're degenerate cases that exist in theory but won't show up on any real blueprint or exam unless someone is trying to trap you. Here is the complete list, straight and unembellished: 1 — Monógono (degenerate, a point. Not a valid polygon.)

2 — Digon (degenerate, a line segment. Also not valid.) 3 — Triângulo 4 — Quadrilátero

5 — Pentágono 6 — Hexágono 7 — Heptágono

8 — Octógono 9 — Eneágono (also called nonágono in some textbooks — both are correct, but "eneágono" is far more common in Brazil) 10 — Decágono

11 — Hendecágono 12 — Dodecágono 13 — Tridecágono

14 — Tetradecágono 15 — Pentadecágono 16 — Hexadecágono

17 — Heptadecágono 18 — Octodecágono (some sources write "octdecágono" — same thing) 19 — Eneadecágono

20 — Icoságono The pattern becomes predictable after 12. Greek numerical prefixes are chained together: "penta" (5) + "decagon" = pentadecagon (15). "Hexa" (6) + "decagon" = hexadecagon (16). It's mechanical once you see it. I ran into a specific problem last year when a client sent me a floor plan where a room was labeled as a "nonágono." The actual shape had 9 sides, sure, but three of those sides were not straight — two were curved arcs and one was a diagonal cut that didn't meet the others at a proper vertex. I flagged it as non-compliant because a true nonagon requires nine straight sides meeting at nine distinct vertices with no curves. They ended up redrawing it. The lesson here is that the name describes the ideal form, and real-world drawings frequently violate that ideal without anyone noticing.

Another thing most people miss: the sum of interior angles follows a simple formula — (n-2) × 180° — but nobody seems to remember which "n" refers to. It's the number of sides, obviously, but on a test they'll often give you the number of diagonals instead and expect you to reverse-engineer it. The diagonal formula is n(n-3)/2. If you know both, you can work backwards from either piece of information. I once spent twenty minutes on a call with a surveyor who kept mixing up the two formulas and getting the wrong angle sum for a decagon. He was subtracting 3 from 10 and dividing by 2, then plugging that result into the angle formula. Wrong path entirely. There's also a practical limitation worth noting. Beyond the icosagon (20 sides), the names get obscure fast. A 21-sided figure is an icosikaipegon, and most people — including engineers I've worked with — just call it a "21-gon" and move on. The naming system doesn't break, but its usefulness does. For anything above 20 sides, describing it numerically is faster and less prone to error than spelling out the full Greek-derived name. If you need a downloadable reference sheet, the Brazilian Ministry of Education publishes a geometry appendix in their secondary education curriculum guidelines that includes this exact table, though it stops at 12 sides. For the full 1-to-20 list, your best bet is a custom export from a CAD library or a straightforward spreadsheet you build yourself. Third-party sheets floating around the internet tend to have typos — I've seen "heptadecágono" spelled with an extra "t" more times than I care to admit.