The big picture
Ten ideas about lines and angles fit on one screen.
| Idea | Remember |
|---|---|
| Cross or never | Two lines meet at one point, or never meet. No third case. |
| Parallel | Never meeting, and the same gap everywhere, measured square across. |
| Opposite angles | Facing angles at a crossing are equal, however the page turns. |
| On a line | Pieces on a line add to 180°. All round a point, 360°. |
| The transversal | One line across two makes eight angles. Three named pairs: corresponding, alternate, co-interior. |
| Only when parallel | Those three pair facts hold only on parallel lines. One degree of tilt breaks them. |
| Chase the angle | Walk from a known angle to an unknown one, naming each fact used. |
| Perpendicular | Two lines that cross at a right angle are perpendicular. If one of the four angles is 90°, all four are. A small square marks the corner. |
| Parallels from square corners | On a flat page, two lines perpendicular to one line are parallel. Mark parallel lines with arrows. Mark a right angle with a small square. |
| A bend between parallels | A path bends at O between two parallel lines. Its two ends lie on the same side of O. At each end, mark the angle that opens towards O. Draw a parallel through O. Alternate angles are equal. So the bend is the sum of the two marked angles. |
In your book, see Part I, Chapter 5, Parallel and Intersecting Lines. Try its questions on lines and angles.