What Is the Name of a Shape With Nine Sides?

A nine-sided polygon is called a nonagon, also known as an enneagon.

A geometry quiz stares back at you: nine straight sides, nine corners. The name hovers just out of reach. Most people have heard of pentagons and hexagons, but when the side count reaches nine, the vocabulary gets fuzzy.

The answer is simpler than you might think. A nine-sided shape is called a nonagon, and it has a second name—enneagon—depending on whether you prefer Latin or Greek roots. This article covers what a nonagon is, its key geometric properties, and how it fits into the family of polygons.

What Is a Nonagon? Definition and Word Origins

A nonagon is any polygon with nine sides and nine vertices (corners). It belongs to the same family as triangles, squares, and pentagons—all closed two-dimensional shapes made of straight line segments.

The name “nonagon” comes from two languages mixed together. The Latin prefix “nona-” means ninth, while the Greek suffix “-gon” means angle or corner. That blend is why some math purists prefer the alternative name “enneagon,” which stays entirely Greek: “ennea” for nine, plus “gon.” Both words refer to the exact same shape.

You might also see “9-gon” in shorthand notation. This is common in geometry problems where the side count is written plainly rather than spelled out.

Why Two Names for the Same Shape?

If you have ever heard someone say “enneagon” and wondered if they meant a different shape, you are not alone. The two names exist because mathematicians borrowed from both Latin and Greek when naming polygons. This historical quirk creates confusion, but it also gives you a handy memory trick.

• Nonagon (Latin prefix): “Nona-” comes from Latin for ninth. It follows the pattern seen in nonary (base-nine) systems. This is the more common name in modern math classes.
• Enneagon (Greek prefix): “Ennea-” is Greek for nine, the same root that gives us enneagram (a nine-pointed figure). Some textbooks, especially older ones, prefer this term.
• 9-gon (numeric alias): When the side count is long or the language root is unclear, simply writing “9-gon” avoids any debate. It is unambiguous and widely accepted.
• Regular vs. irregular: Both nonagon and enneagon can describe regular shapes (all sides and angles equal) or irregular ones (unequal sides and angles). The name does not imply perfection.

So next time you see a nine-sided figure, you can call it a nonagon and be correct. Using “enneagon” is also right, just less common in everyday conversation.

Key Geometric Properties of a Nonagon

A regular nonagon—the kind with all sides and angles identical—has several neat mathematical properties. The sum of its interior angles is always 1260 degrees, regardless of whether the shape is regular or irregular.

To find one interior angle of a regular nonagon, divide the sum by nine. The formula (180n – 360) / n with n=9 gives 140 degrees per angle. That makes each interior corner slightly wider than a right angle (90°) but narrower than a straight line (180°). The exterior angle, the turn you make when walking around the shape, is 40 degrees. The word “enneagon” traces to the Greek “ennea” for nine—the same root Study.com highlights in its enneagon Greek origin explanation.

A regular nonagon also has nine lines of symmetry. Each line passes through one vertex and the midpoint of the opposite side. And if you draw all possible diagonals—lines connecting non‑adjacent vertices—a nonagon contains 27 of them. That is far more than a hexagon (9) or an octagon (20).

Notice the pattern: each time you add a side, the interior angle sum increases by 180°. That is because you can divide any polygon into triangles, and each triangle contributes 180°.

How to Identify a Nonagon – Three Simple Checks

When you see a shape and suspect it might be a nonagon, you do not need to count sides one by one. These three steps help confirm quickly.

1. Count the sides or vertices. A nonagon has exactly nine of each. If you see eight sides, it is an octagon; ten sides, a decagon. Counting is the most reliable method.
2. Check the angle feel. In a regular nonagon, each interior angle is 140 degrees—noticeably wider than a square’s 90° but still less than a straight line. An irregular nonagon will have varying angles, but the sum will always be 1260°.
3. Look for nine lines of symmetry (regular only). If the shape is perfectly symmetrical and appears star-like with nine evenly spaced points, it is almost certainly a regular nonagon. If it is lopsided, it is an irregular nonagon (still a nonagon, just not a perfect one).

These checks work for any nine-sided polygon, whether you call it a nonagon or enneagon. The geometry is the same.

Real‑World Examples and Construction Tips

You might not see a nonagon every day, but they do appear. Some coins, architectural details, and even certain musical symbols (like the nine‑pointed star in the Baháʼí faith) use nine‑fold symmetry. Building a regular nonagon by hand with only a compass and straightedge is famously impossible—it is not a constructible polygon in classical Euclidean geometry.

However, you can approximate one: start with a circle, mark the center, then use a protractor to make 40‑degree angles (the exterior angle). Draw nine equal chords around the circle. The result will be close enough for most practical purposes. Per nonagon definition on Wikipedia, a regular nonagon has nine equal sides and nine equal angles.

For students, the nonagon is a good test of understanding polygon formulas. If you can handle nine sides, you can handle any number—just plug n into the formulas.

That 40‑degree exterior angle is what makes the regular nonagon look so different from a circle’s 360‑degree full turn. Walking around the shape, you turn 40° at each vertex—eight turns total (since the ninth brings you back to start).

The Bottom Line

A nine‑sided polygon is called a nonagon (or enneagon, or 9-gon). Its interior angles sum to 1260°, each interior angle is 140° in the regular version, and it has 27 diagonals. The shape sits between the octagon (8 sides) and decagon (10 sides) in the polygon family, and its two names reflect the mixed language roots of geometry. For anyone studying polygons, the nonagon is a perfect example of how side count drives all the key numbers—angles, diagonals, and symmetry—without needing to memorize endless formulas.

If you are working through a geometry curriculum and the nonagon formula feels tricky, having a tutor or using the official curriculum standards from your school’s math department can help you practice the (180n – 360) / n calculation with concrete examples like a nonagon until it sticks.