Basic geometry: language and labels

Notes from Khan Academy video Basic geometry: language and labels | Introduction to Euclidean geometry | Geometry | Khan Academy

  • Word geometry is made up of 2 components: "geo" and "metry":
    • "Geo" - earth
    • "metry" - measurement
    • Therefore, geometery is about how shapes and space (things we see) related to each other.


  • A Plane is a flat surface that has 2 dimensions: width and height and goes on infinity in either direction.


  • A Point refers to a dot on that plane and is the most basic geometric idea.

    points on a plane

    • Points are commonly labelled using upper case letters: A, B, C, D etc
    • Point is said to be 0-dimension: it doesn't provide any direction information.

Line Segment

  • A line that connects 2 points, is referred to as a Line Segment

    line segment examples

    • A line segment is described using its endpoints: the points that it is connected by.
    • In math notation, a line is drawn over the endpoints to denote that it's a line segment.


    • The endpoints of a line segment can be inversed:

      AB=BA\overline{AB} = \overline{BA}

    • Is one-dimension - you can move back and forth along it.


  • A line that starts from one point that continues on infinitely in one direction is called a Ray

    ray example

    • The order of a ray is meaninful, unlike a line segment, as it describes the direction of the ray:

      ABBA\overrightarrow{AB} \neq \overrightarrow{BA}

    • The starting point is referred to the endpoint.


  • A line that continues infinitely in either direction is formally called a Line.

    line example

    • A line is described by the endpoints it intersects, typically in alphabetical order though the order does not matter.

      AB=BA\overleftrightarrow{AB} = \overleftrightarrow{BA}

Collinear points

  • If you have a line segment and introduce a point in the middle, you could describe all 3 points as "Colinear" - they sit on the same line.

    colinear points example

  • In the above picture, if the distance between XZXZ was the same as the distance between ZXZX, then you would refer to Z as the midpoint of the line segment XYXY.