How Are Polygons Named? | From Triangle To Icosagon

Polygon names look tricky at first, but the pattern is cleaner than it seems. Once you know that the front part of the word tells the number and the ending -gon points to angles or sides, names like pentagon, hexagon, and dodecagon stop feeling random.

That pattern helps in class, on homework, and in everyday reading. A stop sign is an octagon. A five-sided badge is a pentagon. A 12-sided design on a tile is a dodecagon. You do not need to memorize every shape name one by one. You can build many of them from parts.

What A Polygon Name Is Made Of

A polygon is a closed flat shape made from straight line segments. The naming part starts with two building blocks. One part gives the count. The other part gives the shape family.

The Number Prefix

The prefix comes from Greek number words in most cases. That is why three sides becomes tri-, five becomes penta-, six becomes hexa-, and eight becomes octa-. Put that prefix at the front, and you have the start of the shape’s name.

The Ending “-gon”

The ending -gon ties the word back to angle. In school math, we use the name to stand for the whole shape, not just its corners. So when you hear hexagon, you can read it as “six-angle shape,” which matches a six-sided polygon.

That same setup gives you a neat reading trick:

• Tri + gon = three-sided polygon
• Penta + gon = five-sided polygon
• Deca + gon = ten-sided polygon
• Dodeca + gon = twelve-sided polygon

Once you see the parts, long names stop looking dense. They become number words in disguise.

How Are Polygons Named? The Pattern That Stays The Same

The usual rule is plain: count the sides, match that number to a Greek-style prefix, then add -gon. That is why a seven-sided polygon is a heptagon and a nine-sided polygon is a nonagon or enneagon, depending on the naming style being used.

In most school settings, the names from three through ten show up the most. Those are the ones worth getting comfortable with first. After that, names above ten follow the same habit, just with longer prefixes.

The Names You’ll See Most Often

These are the forms that turn up again and again in math books, worksheets, and diagrams:

• 3 sides: triangle
• 4 sides: quadrilateral
• 5 sides: pentagon
• 6 sides: hexagon
• 7 sides: heptagon
• 8 sides: octagon
• 9 sides: nonagon
• 10 sides: decagon

One detail stands out right away. Four sides is often called a quadrilateral, not a tetragon, even though tetragon exists. That is one of the few places where everyday classroom usage leans toward a different standard name.

Number Of Sides	Prefix Or Root	Common Polygon Name
3	tri-	triangle
4	quadri- / tetra-	quadrilateral or tetragon
5	penta-	pentagon
6	hexa-	hexagon
7	hepta-	heptagon
8	octa-	octagon
9	nona- / ennea-	nonagon or enneagon
10	deca-	decagon
11	hendeca- / undeca-	hendecagon or undecagon
12	dodeca-	dodecagon

Why Some Polygon Names Sound Different

Most names fit the rule neatly, but a few have two accepted forms or one form that shows up more often in class. That is normal. Geometry pulls from old Greek roots, later Latin spellings, and long teaching habits.

Britannica’s definition of a polygon keeps the math side clear: polygons are closed figures made of line segments. MathWorld’s note on the word “polygon” traces the name back to Greek roots meaning many and angle. That explains why so many names sound related even when the spelling shifts a bit.

Common Double Names

Some side counts carry two names that teachers may treat as correct:

• 9 sides: nonagon or enneagon
• 11 sides: undecagon or hendecagon
• 4 sides: quadrilateral in general use, tetragon in stricter naming systems

That does not mean the system is messy. It only means language has history. A class handout might prefer one form, while a math dictionary lists both.

Why Triangle Does Not End In “-gon”

Triangle is the famous outlier. It comes from a different naming stream and has stayed in daily use for centuries. You could say trigon in a technical sense, but almost nobody does in basic geometry. The same thing happens with quadrilateral. Usage wins.

If you want the safe classroom move, use triangle and quadrilateral for 3 and 4 sides, then switch to pentagon, hexagon, heptagon, and so on after that.

Polygon Names After Ten Sides

Past ten, the system keeps going. The names just get longer. Once you know the number roots, you can read many of them without seeing them before.

How Compound Names Work

For some numbers, the front part stacks pieces together. Eleven and twelve are the most common cases students meet after ten. Merriam-Webster’s word history for dodecagon shows the Greek root behind the 12-sided form, which is why the pattern still feels familiar even in longer names.

Here is the idea in plain terms:

• 11 often becomes hendecagon or undecagon
• 12 becomes dodecagon
• 20 becomes icosagon

Once the side count climbs high enough, many teachers stop expecting you to memorize every name. They care more that you can spot the prefix and connect it to the right number.

Sides	Name You May See	What To Notice
11	undecagon / hendecagon	Two common forms exist
12	dodecagon	Shows the Greek root for twelve
13	tridecagon	Built from three + ten
14	tetradecagon	Uses tetra for four
15	pentadecagon	Uses penta for five
16	hexadecagon	Uses hexa for six
17	heptadecagon	Uses hepta for seven
18	octadecagon	Uses octa for eight
19	enneadecagon	Built from nine + ten
20	icosagon	A common higher-count name

Regular And Irregular Shapes Keep The Same Name

A polygon’s name comes from the number of sides, not from whether all sides match. That means a regular pentagon and a lopsided five-sided figure are both pentagons. The word tells you the side count first.

Extra words do the rest of the job:

• Regular means all sides and all angles match.
• Irregular means they do not.
• Convex means no corner points inward.
• Concave means at least one corner points inward.

So the full label might be “regular octagon” or “concave decagon.” The base name still comes from counting sides.

Easy Ways To Remember Polygon Names

You do not need a giant chart in your head. A few habits make the naming system stick.

Start With The Reliable Core Set

Learn 5 through 10 well: pentagon, hexagon, heptagon, octagon, nonagon, decagon. Those appear often, and each one reinforces the prefix pattern.

Use Real Objects

An octagonal stop sign, a hexagonal tile, or a pentagonal badge can pin the word to a shape you have already seen. That makes recall easier than rote memorization.

Break Long Names Into Chunks

Dodecagon looks long until you read it as dodeca + gon. The same trick works with hexadecagon and octadecagon. Long words become small parts again.

Naming Polygons Becomes Predictable Once You Count

If you can count the sides, you are most of the way there. Match the number to the prefix, add -gon, and watch for a few familiar classroom forms like triangle and quadrilateral. That is the whole engine behind polygon names.

After a while, the names stop feeling like vocabulary and start feeling like code you can read on sight. Count first. Build the word second. The shape tells you the answer.