Skill: basic-vocabulary
Points, Lines, Planes & Angle Vocabulary
Every geometry proof, diagram, and formula is built from five ideas: point, line, ray, segment, and plane —
plus the language of angles. Learn to name them, draw them, and write them in correct notation.
1 The five building blocks
A point marks a location. A line is straight and extends forever in both directions.
A ray starts at one point and extends forever in one direction. A segment is the part of a line
between two endpoints. A plane is a flat surface extending forever in all directions.
Point
An exact location with no size. 0 dimensions.
Notation: capital letter — A, B, P
Line
Straight path extending forever in both directions. 1 dimension.
Notation: ↔AB or a lowercase script letter
Ray
Starts at an endpoint, extends forever in one direction. The endpoint is always named first.
Notation: →AB (starts at A, goes through B)
Segment
The part of a line between two endpoints — it has a measurable length.
Notation: AB (overline)
Plane
A flat surface extending forever in all directions. 2 dimensions.
Notation: script capital letter — plane M
Angle
Two rays sharing a common endpoint (the vertex). The rays are the sides.
Notation: ∠ABC (vertex B in the middle) or ∠B
2 Measuring and classifying angles
Angles are measured in degrees (°). The measure m∠ABC is a number; the name ∠ABC is the figure.
Every angle falls into one of four families.
A protractor has two scales. If the angle’s first ray points left, read the scale that starts at 0° on the left.
If it points right, read the scale that starts at 0° on the right. Guessing the scale gives you the supplement (e.g., 60° vs. 120°) — the most common protractor error.
3 Worked examples
Four quick applications of the vocabulary: naming a figure, classifying an angle, writing notation, and reading a protractor.
A figure starts at point A, passes through point B, and has an arrowhead past B (one direction only).
One endpoint and one arrowhead → it is a ray, named →AB (endpoint A named first).
An angle measures 135°.
135° is between 90° and 180°, so it is obtuse.
A straight figure extends forever in both directions through points A and B.
Arrows on both ends → a line, written ↔AB. (Writing AB would wrongly claim it stops at the endpoints.)
An angle’s first ray points left along the 0° line; the second ray crosses the 60° mark on the left-starting scale
(the right-starting scale says 120° there).
The angle opens from the left, so read the left scale: 60°.
4 Common mistakes
Two traps from this skill, each with the wrong version and the fix.
Wrong: calling ↔AB “segment AB” because points A and B are marked on it.
Right: the arrows decide, not the labeled points. Arrows on both ends → line ↔AB;
no arrows → segment AB.
Wrong: an angle opening from the left is read as 120° off the right-starting scale.
Right: always start at 0° on the side the angle opens from. Sanity-check: acute angles read under 90°, obtuse over 90°.
5 Quick checks
Try these yourself, then reveal the answer.
6 Key points
Remember
- Line ↔AB: arrows both ends, goes forever. Ray →AB: one endpoint, one arrow. Segment AB: two endpoints, measurable length.
- Point = 0 dimensions, line = 1, plane = 2.
- Angle classes: acute (0°–90°), right (= 90°), obtuse (90°–180°), straight (= 180°).
- Write angles as ∠ABC with the vertex in the middle; m∠ABC is its measure.
- Protractor rule: start at 0° on the side your angle opens from; sanity-check acute vs. obtuse.