Everything in geometry starts with a simple idea: a point. A point isn't a thing you can hold or measure. It's just a location. It has no size, no width, and no depth. We usually draw it as a dot, but remember that the dot is just a symbol for an exact position.

Now, imagine connecting an infinite number of points in a perfectly straight path. That path would stretch forever in both directions. This is a line.

A line is just one way to arrange points. We can also talk about parts of a line.

When two rays share a common endpoint, they form an angle. That shared endpoint is called the vertex. We measure angles in degrees, which tells us how much one ray is "opened" or rotated away from the other.

Angles come in a few basic types, depending on their size.

Lines don't always meet. Sometimes they run alongside each other forever without ever touching. These are called parallel lines.

The opposite of parallel is when lines intersect, or cross each other. A special type of intersection is when two lines cross to form perfect right angles ($90^\circ$). These are called perpendicular lines.

When any two lines intersect, they create four angles. The angles opposite each other at the intersection are called vertical angles, and they are always equal.

Time to review these core concepts before we move on.

Let's check your understanding with a few questions.

With these building blocks—points, lines, and angles—we can start constructing and analyzing all the shapes and figures in the world of geometry.