Schooling · Class 6 · Mathematics · Lines and Angles · Chapter on a page
CBSE Class 6 · Mathematics · NCERT Ganita Prakash
Lines and Angles: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 26 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- point
- line
- line segment
- ray
- angle
- degrees
- right angle
- Formulas and facts
- Right angle
- Straight angle
- Complete angle
- Acute angle
- Obtuse angle
- vertex
- Watch out for
- Drawing a line segment with arrows…
- Thinking that a bigger-looking angle…
- Reading the wrong scale on a…
- Confusing rays and line segments
- Forgetting to put the vertex at the…
What this chapter is about
This chapter introduces you to the world of lines and angles. You will learn what a point, a line and a line segment are. You will also learn about rays and how they are different from lines.
Key ideas
- A point is a dot that shows an exact position. It has no length, width or height.
- A line goes on forever in both directions. We show it with arrows on both ends.
- A line segment is a part of a line with two endpoints. It has a fixed length.
- A ray starts at one point and goes on forever in one direction. The sun's rays are a good example.
- An angle is formed when two rays share the same starting point. That shared point is called the vertex.
- Angles are measured in degrees. The symbol for degrees is °.
- A right angle measures exactly 90°. The corner of a book or a door makes a right angle.
- An acute angle is smaller than a right angle, meaning it is less than 90°.
- An obtuse angle is larger than a right angle but smaller than a straight angle, meaning it is between 90° and 180°.
Formulas and facts to remember
- Right angle = 90° (the angle at the corner of a square)
- Straight angle = 180° (the angle on a straight line)
- Complete angle = 360° (one full turn, like a clock hand going all the way around)
- Acute angle: any angle more than 0° but less than 90°
- Obtuse angle: any angle more than 90° but less than 180°
- Two rays forming an angle share a common point called the vertex.
Common mistakes
- Drawing a line segment with arrows on both ends → A line segment has dots at both ends (endpoints), not arrows.
- Thinking that a bigger-looking angle is always more degrees → Always measure with a protractor; the length of the arms does not decide the angle.
- Reading the wrong scale on a protractor → Check whether your angle opens from 0° on the inner scale or the outer scale.
- Confusing rays and line segments → A ray has one endpoint and goes on forever in one direction, while a line segment has two endpoints.
- Forgetting to put the vertex at the centre of the protractor → The vertex must sit exactly at the centre mark when measuring.
Quick revision
- A point shows position. A line has no end. A line segment has two ends. A ray has one end.
- Angle = two rays sharing one vertex.
- Right angle = 90°, Acute angle < 90°, Obtuse angle > 90° but < 180°.
- A full turn = 360°, a straight line = 180°.
- Use a protractor to measure angles in degrees.