The idea, step by step
Straight or curved?
A straight edge does not bend. A curved boundary bends as you follow it. A circle is curved all the way around. Some shapes, such as a semicircle, have both a straight edge and a curved part.
Trace slowly when a shape has both kinds of boundary. Say which part you are describing rather than calling the whole shape straight or curved.
Across from each other
Opposite sides are across the shape from each other. They do not share a corner. In a rectangle, AB is opposite CD, and AD is opposite BC.
Parallel straight sides stay the same distance apart. In this rectangle, each pair of opposite sides is parallel. Opposite sides in other four-sided shapes are not always parallel.
A common trap
Any two sides that do not touch are parallel.
Instead: Not touching within the drawing is not enough. Parallel straight sides keep the same gap if their lines are continued.
See the thinking
Find an opposite side
In rectangle ABCD, which side is opposite AB?
- Find AB at the top.
- Look across the rectangle to the bottom edge.
- That edge joins C and D.
Notice a parallel pair
Which labelled edges of the trapezium are parallel?
- Compare the direction of the top and bottom edges.
- Both are horizontal, and the gap between their straight lines stays the same.
- The sloping side edges do not form another parallel pair.
Check your understanding
In a rectangle, does AB share a corner with its opposite side CD?
Show answer
No.
Opposite sides are across from one another and do not share a corner.
Your turn
6 questions · Work at your own pace.
Read the questions online
- Is the boundary of a circle straight or curved?
- A semicircle is made by cutting a circle straight across its centre. Name the two kinds of boundary it has.
- In rectangle ABCD, name the side opposite AD.
- Name a side that shares a corner with AB.
- Name the parallel pair in the trapezium.
- Sam draws two sloping lines that get closer together. He says they are parallel because they have not met yet. Is he right? Explain.
Read the worked solutions
- Is the boundary of a circle straight or curved?
It bends all the way around.
Curved. - A semicircle is made by cutting a circle straight across its centre. Name the two kinds of boundary it has.
The cut is straight. The remaining half of the circle is curved.
One straight edge and one curved part. - In rectangle ABCD, name the side opposite AD.
AD is on the left. BC is across from it on the right.
BC. - Name a side that shares a corner with AB.
AD shares corner A; BC shares corner B. Either answer is correct.
AD or BC. - Name the parallel pair in the trapezium.
The horizontal edges keep the same separation.
The top and bottom edges. - Sam draws two sloping lines that get closer together. He says they are parallel because they have not met yet. Is he right? Explain.
Their gap changes, so they are not parallel even before they meet.
No.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum Mathematics
Sides and spatial features; straight/curved/opposite/parallel; quarter/half/three-quarter/full turns.
Official curriculum source ↗