- Topics At a Glance
- The Basics of Circles
- Center and Radius
- Central Angles
**Arcs****Arc Measure vs. Length**- Arc Length and Circumference
- Angles and Arcs
- Chords
- Angles and Chords
- Arcs and Chords
- Inscribed Angles
- Inscribed Angle Theorem
- Tangents and Secants
- Perpendicular Tangent Theorem
- Tangents and Multiple Circles
- Circles on the Coordinate Plane
- Equations of Circles
- Circles and Lines

The measure of a central angle tells us something about its intercepted arc: how "much of a circle" that arc is. The **measure **(sometimes called the **degree measure**)** of an arc **is equal to the measure of the central angle that intercepts that arc.

In the figure above, both ∠*AOB* and arc *AB* have measure 90°. Both ∠*COD* and arc *CDE* have measure 180°.

Every arc falls into exactly one of the following categories, depending on its measure.

1. An arc with measure *less than *180° is a **minor arc**.

2. An arc with measure *greater than *180° is a **major arc**.

3. An arc with measure *equal to *180° is a **semicircle**.

Note that a major arc's buddy is always a minor arc, and vice versa. A semicircle's buddy is always a semicircle. They're basically twins, like Lindsay Lohan and…uh…Lindsay Lohan.

An arc has a measure of 73°. Is its buddy a minor arc, a major arc, or a semicircle arc? What is its measure?

Since 73° is less than 180°, we know that the first arc is a minor one. Since the buddy of a minor arc is always a major arc, we know right off the bat that our arc is major. We can find its measure by subtracting our original arc from the total measure of the circle. That gives us 360° – 73° = 287°. Our arc has a measure of 287°, much larger than 180° and definitely major.

In addition to its measure, arcs have another property called **arc length**. The length of an arc is the distance a bug would have to crawl from one endpoint of the arc to the other, while staying on the circle the whole time.

It's important to remember that arc *length* and arc *measure* are NOT the same.

If you're ever unsure whether you're dealing with arc length or arc measure, check the units. Arc measure is almost always given in units of degrees. Arc length, on the other hand, is a length, and so it may be given in units of meters, feet, inches, kilometers, or unhelpfully, "units," but certainly *not* degrees. (Don't be afraid of units. They're there to help you.)

It's important to be able to tell when two things are different and when they are the same. It's a good skill to have, regardless of how you feel about circles, geometry, or Sesame Street.