In Grade 2 you described shapes by their sides and corners. Geometry gives the parts of shapes exact names, starting with points and straight paths.
Key idea
A point is an exact spot, named by a letter. A segment $\overline{AB}$ is the straight path between two endpoints. A ray $\overrightarrow{CD}$ starts at an endpoint C and goes on forever through D. A line $\overleftrightarrow{EF}$ goes on forever in both directions.
Why it works
The arrowheads in a drawing mean "keeps going." A segment has no arrows because it stops at both ends. A ray has one arrow, so its name must say where it starts: $\overrightarrow{CD}$ starts at C, while $\overrightarrow{DC}$ would start at D and point the other way. A line has no start, so $\overleftrightarrow{EF}$ and $\overleftrightarrow{FE}$ name the same line.
Worked example: Name a ray
A ray starts at point P and passes through point Q. Name it.
- The endpoint comes first: P.
- Then a point it passes through: Q.
Answer: $\overrightarrow{PQ}$
Worked example: Points on a line
Points A, B and C lie on a line in that order. Name two different rays that start at B.
- From B, one ray goes toward C and the other toward A.
- Each name starts with B.
Answer: $\overrightarrow{BC}$ and $\overrightarrow{BA}$
Common mistake
Writing a ray's letters in the wrong order, like $\overrightarrow{QP}$ for a ray that starts at P. The first letter is always the endpoint. Check: does the arrow in your name point the same way as the arrow in the drawing?