Angle Basics – Vertex, Arms, and Naming an Angle
An angle is formed whenever two rays (or line segments) share a common starting point. That shared point is called the vertex, and the two rays are called the arms of the angle. The size of the angle measures how much one arm has “turned” away from the other, around the vertex.
The word “angle” comes from the Latin angulus, meaning “corner” – a fitting origin, since a corner is exactly where two edges meet at a point, just like the arms of an angle meet at its vertex. Getting the vocabulary (vertex, arms) and the naming convention solid now makes every angle topic in the rest of this section far easier to follow.
Vertex and Arms
An angle has a vertex (the shared point) and two arms (the two rays extending from it). Angles are usually named with three letters, such as “angle ABC,” where the middle letter is always the vertex.
The middle letter names the vertex: Q.
Real-Life Application
- Scissors: the pivot point of a pair of scissors is the vertex of the angle between its blades.
- Clock hands: the centre of a clock is the vertex of the angle between the hour and minute hands.
- Book covers: opening a book creates an angle with the spine as the vertex.
Key Takeaways
- An angle is formed by two arms meeting at a vertex.
- Angles are named with three letters, with the vertex always in the middle.
- This vocabulary is the foundation for every other angle topic in this section.
Practice: Angle Basics
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