There are so many shapes around us made up of lines and curves like line segments, angles, triangles, polygons and circles etc. These shapes are of different sizes and measures.
A line segment is a fixed part of the line, so it must have some length. We can compare any line segment on the basis of their length.
1. Comparison by Observation
We can tell which line segment is greater than other just by observing the two line segments but it is not sure.
Here we can clearly say that AB > CD but sometimes it is difficult to tell which one is greater.
2. Comparison by Tracing
In this method we have to trace one line on paper then put the traced line segment on the other line to check which one is greater.
3. Comparison using Ruler and a Divider
We can use a ruler to measure the length of a line segment.
Put the zero mark at point A and then move toward l to measure the length of the line segment, but it may have some errors on the basis of the thickness of the ruler. This could be made accurate by using a Divider.
We can understand the concept of right and straight angles by directions.
There are four directionsNorth, South, East and West.
There are so many other types of angles which are not right or straight angles.
By observing an angle we can only get the type of angle but to compare it properly we need to measure it.
Steps to measure an angle using protractor
Step 1: Place the protractor on the angle in such a way that the midpoint of protractor comes on the vertex B of the angle.
Step 2: Adjust it so that line BC comes on the straight line of the protractor.
Step 3: Read the scale which starts from 0° coinciding with the line BC.
Step 4: The point where the line AB comes on the protractor is the degree measure of the angle.
Hence, ∠ABC = 72°.
If two lines intersect with each other and form an angle of 90° then they must be perpendicular to each other.
Here AB and MN are intersecting at point N and form a right angle. We will write it as AB ⊥ MN or MN ⊥ AB Reads as AB is perpendicular to MN or MN is perpendicular to AB.
If a perpendicular divides another line into two equal parts then it is said to be a perpendicular bisector of that line.
Here, CD is the perpendicular bisector of AB as it divides AB into two equal parts i.e. AD = DB.
Triangle is a polygon with three sides. It is the polygon with the least number of sides. Every triangle is of different size and shape. We classify them on the basis of their sides and angles.
1. Classification on the basis of sides
2. Classification on the basis of Angles
A polygon with four sides is called Quadrilateral.
Any closed figure made up of three or more line segments is called Polygon.
We can classify the polygons on the basis of their sides and vertices 
The solid shapes having three dimensions are called 3D shapes.
Some of the 3D shapes around us
No. of Faces, Edges and Vertices in some common 3 D shapes
92 videos347 docs54 tests

1. How do you measure line segments? 
2. What is the difference between a "right" angle and a "straight" angle? 
3. What are acute, obtuse, and reflex angles? 
4. How do you measure angles? 
5. What are perpendicular lines and perpendicular bisectors? 

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