Lines and Angles  Basic Terms
A line is defined as a row of closely spaced dots that extends infinitely in two directions. A line has only one dimension, that is length. A horizontal mark drawn on a piece of paper can be considered as an example of a line. In geometry, an angle can be defined as a figure created by two rays that meet at a common endpoint. They are measured in degrees, using a protractor. All geometry shapes have angles and lines in them. Let us learn more about the various types of lines and angles along with a few solved examples and practice questions.
1.  Introduction to Lines 
2.  Types of Lines 
3.  Introduction to Angles 
4.  Types of Angles 
5.  Solved Examples 
6.  Practice Questions 
7.  FAQs on Lines and Angles  Basic Terms 
Introduction to Lines
A line is a onedimensional figure that extends in both directions infinitely without any width. It is made up of an endless number of points close to each other. Euclid denotes the line as a breadthless length. In a cartesian plane, it is denoted by the linear equation ax + by = c. Observe the line shown in the figure given below.
Rays
Rays are the lines with one end as the start point and the other end going to infinity. They extend in one direction without ending. When two rays are joined end to end, they form an angle.They are represented as \( \overrightarrow{\mathrm{PQ}}\)
Line Segment
When a line has two endpoints, it is known as a line segment. The length of a line segment can be measured and it is written as \(\overline{AB}\)
Types of Lines
Lines can be classified into different types depending upon their properties. The table below shows the different types of lines that are categorized based on their properties.
Type of Lines  Description  Illustration 
Horizontal Line 
These lines are parallel to the xaxis and perpendicular to the yaxis 

Vertical Line  These lines are parallel to the yaxis and perpendicular to the xaxis 

Parallel Lines  When two straight lines move alongside and maintain a constant distance between each other up to infinity, they are known as parallel lines. The symbol  is used to represent parallel lines. 

Perpendicular Lines  When two lines intersect each other at an angle of 90°, they are known as perpendicular lines. Perpendicular lines are denoted by the symbol of ⊥  
Transversal Lines  When a line intersects two lines at different respective points, then it is known as a transversal line. 

Introduction to Angles
Types of Angles
Angles can be categorized into different types based on their measurements. Angles are generally of 6 types:
 Acute angle: If the measure of an angle is less than 90^{∘} then it is known as an acute angle.
 Obtuse angle: If the measure of an angle is greater than 90^{∘} but less than 180^{∘}, then it is known as an obtuse angle.
 Right angle: If the measure of an angle is exactly equal to 90^{∘} then it is known as a right angle.
 Straight angle: If the measure of an angle is 180^{∘} then it is known as a straight angle.
 Reflex angle: If the measure of an angle is greater than 180^{∘} but less than 360^{∘}, then it is known as a reflex angle.
 Complete angle: If the measure of an angle is 360^{∘} then it is known as a complete angle.
The table given below shows the different types of angles that are categorized based on their measurements.
Type of Angles  Measurements  Illustration 

Acute Angle 
Here, (∠ABC = 40^{∘}) < 90^{∘} 

Right Angle 
Here, ∠ABC = 90^{∘} 

Obtuse Angle 
Here, 90^{∘} < (∠ABC = 117^{∘}) <180^{∘} 

Straight Angle 
Here, ∠AOB = 180^{∘} 

Reflex Angle 
180^{∘ }< (Reflex Angle = 330^{∘}) < 360^{∘} 

Complete Angle 
Angle = 360^{∘} 

Related Topics
Check out these interesting articles to learn more about the properties of lines and angles, and their related topics.
Important Notes:
Here is a list of a few important points that should be remembered while studying lines and angles:
 All geometry shapes have angles and lines in them.
 A line is a onedimensional figure, with no breadth, and that extends in both directions infinitely.
 These are the lines with one end as the start point and the other end going to infinity. These are used to form angles.
 Angles are formed when two rays intersect at a point.
Solved Examples

Example 1: If the measure of an ∠AOB is 57°. Find the measure of the angle which is the reflex angle of this angle.
Solution:
The addition of an angle and its reflex angle sums up to 360°.
Therefore,
AOB + Reflex ∠AOB = 360°
Reflex ∠ AOB = 360° − 57°
Reflex ∠AOB = 303°
Answer: The reflex ∠ AOB is 303°.

Example 2: Find the value of x in the given figure if AOB is a line, ∠AOC = 4x, and ∠BOC = 2x.
Solution:
The sum of the adjacent linear angles formed by a line is 180°.
Therefore, 4x + 2x = 180°
6x = 180°
x = 180°/6 = 30°
We get the value of x as 30°
Answer: x = 30°.
FAQs on Lines and Angles  Basic Terms
What Are Angles? List the Types of Angles.
Angles are formed when two rays intersect at a point. They are usually measured in degrees and denoted by ∘ (the degree symbol), which is a measure of rotation. The six types of angles are right angle, acute angle, obtuse angle, straight angle, reflex angle, and complete angle,
What Are Lines and Their Types?
A line is a onedimensional figure, with no width, and it extends in both directions infinitely. The different types of lines are  horizontal lines, vertical lines, parallel lines, perpendicular lines, and transversal lines.
What Are Perpendicular Lines?
When two lines intersect each other at an angle of 90°, they are known as perpendicular lines. Perpendicular lines are denoted by the symbol of ⊥.
What Are Horizontal and Vertical Lines?
Horizontal lines are the lines that are parallel to the xaxis and perpendicular to the yaxis. Vertical Lines are the lines that are parallel to the yaxis and perpendicular to the xaxis.
How Are Straight Lines Classified?
Straight lines are classified into horizontal and vertical lines. Apart from these, there are other types of lines such as parallel lines, transversal lines, and perpendicular lines.
Is it True that the Linear Pair of Angles Are Always Congruent?
It is False. Linear pairs of angles are not always congruent. Only when the measure of each of the angles is 90°, a linear pair of angles are considered to be congruent. Linear pairs of angles are always supplementary.