Marks that speak
Arrow marks show parallel lines. A square shows perpendicular lines.
Saying it without words
A drawing can have many lines. Two of them are parallel. How can the picture say so, without any words?
How could a drawing show that two lines are parallel?
What this lesson covers
The idea
In figures, a set of parallel lines is marked with an arrow mark (>), a second set with two arrow marks, and so on; perpendicular lines are marked with a square angle between them.
Saying it without words
A drawing can have many lines. Two of them are parallel. How can the picture say so, without any words?
How could a drawing show that two lines are parallel?
- Write the word “parallel” beside each line
- Put the same small arrow mark on both lines
- Draw both lines thicker
Mark the figure
Here are two horizontal lines and two vertical lines. Mark the figure so that it says which lines are parallel and which are perpendicular.
The language of marks
A set of parallel lines gets the same arrow mark. A second set gets two arrow marks, a third set three, and so on. Perpendicular lines get a square in the corner where they meet.
In figures, a set of parallel lines is marked with an arrow mark (>), a second set with two arrow marks, and so on. Perpendicular lines are marked with a square angle between them.
- Mark | It says
- One arrow mark | A set of parallel lines
- Two arrow marks | A second set of parallel lines
- Square corner | The lines are perpendicular
Notes
In figures, a set of parallel lines is marked with an arrow mark (>), a second set with two arrow marks, and so on. Perpendicular lines are marked with a square angle between them.
Check yourself
The picture marks lines p and q with an arrow each. Which two lines does it say are parallel?
What does the little square at the corner tell us?
Lines a and b have one arrow mark. Lines c and d have two arrow marks. Is line a parallel to line c?
A figure has three sets of parallel lines. How many arrow marks does the third set get?
Answer: 3
The first set gets 1 arrow mark, the second set 2, so the third set gets 3.
- p and r. Line r has no arrow mark, so the picture does not say that r is parallel to p. It crosses p.
- p and q, the two lines with the same arrow mark — correct. Yes! The same arrow mark on both lines says they are parallel.
- q and r. Line r has no arrow mark, so the picture does not say that r is parallel to q. It crosses q.
- Lines l and m are perpendicular: they meet at 90°. — correct. Yes! A square angle between two lines means they are perpendicular.
- Lines l and m are parallel.. Parallel lines are marked with arrow marks. A square is for perpendicular lines.
- Line m is longer than line l.. A square angle says nothing about length. It marks a right angle (90°).
- Yes. Every line with an arrow mark is parallel to every other.. Each set has its own mark: one arrow for one set, two arrows for the next set.
- We cannot tell: arrow marks only show how long the lines are.. Arrow marks do not show length. They show which lines are parallel.
- No. Different marks mean different sets of parallel lines. — correct. Yes! One arrow mark is one set, two arrow marks are another set. Lines in different sets are not parallel.