A line, a straight line, a line segment

Straight lines have names. You will learn to draw a line segment with a ruler and measure it.

A straight line has a name

Anyone can draw a line. But maths tells which kind of line it is.

A curved line wiggles as it likes. A wave, a scribble.

A straight line is a perfectly straight line that has no start and no end. It goes on and on both ways. You can never draw the whole of it — only a bit fits on paper.

A line segment is a straight bit of line from point to point. It has a start and an end. That you can draw — you only need a ruler.

What is it good for? Drawing with a ruler is drawing with care. A plan of a room, a playground drawn on the ground, a frame around a picture — everything starts with a straight line of the right length. And a straight line of the right length is exactly a line segment.

Points are marked with capital letters: A, B, C. The line segment from point A to point B is called AB. And because it has a start and an end, it can be measured. Drawing with a ruler starts right here.

Ema got a new ruler and draws roads between towns on paper. Dot A is home, dot B is school. She joins them with a ruler. “That is line segment AB!” Adam draws the road freehand — wavy. “Mine is nicer.” “But longer,” says Ema. She measures her straight one: 12 cm. Adam’s wave cannot be measured with a ruler. “A straight road is the shortest,” Adam admits. Then they draw a net: school, playground, cake shop. The paper looks like a map. Adam adds a dog too. “He walks wherever he wants — all curved lines.” Ema laughs: exactly, only line segments walk straight.

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When you draw, hold the ruler firmly with the middle of your fingers and run the pencil along the edge. The pencil should be sharp — a thick line lies when you measure.

How to draw and measure a line segment

Drawing a line segment:

Make a dot and write A by it. That is the start.
Make a second dot, write B.
Put the ruler to both dots. Hold it firmly so it does not slip.
加入 the dots with a line along the edge of the ruler. Do not start before the dot and do not go past it.

Done: line segment AB.

Measuring a line segment:

Put the ruler with zero exactly on point A. Not on the edge of the ruler — on zero!
Read the number at point B. For example 7 → line segment AB measures 7 cm.

Drawing a line segment of a given length: you want a line segment 5 cm? Make dot A, put zero on it, make dot B at the number 5 and join.

Comparing line segments is comparing numbers: a line segment 9 cm is longer than a line segment 6 cm. By how much? 9 − 6 = 3 cm. See — even drawing with a ruler ends up counting.

And remember the difference between a straight line and a line segment like this: a straight line is an endless road, a line segment is your path from home to school. It has a start and a finish.

Draw a line segment 6 cm long

Step 1: A dot, the letter A by it.

Step 2: I put the ruler with zero exactly on A.

Step 3: At the number 6 I make a second dot: B.

Step 4: I hold the ruler and join A with B.

Check: I put the ruler on again, zero on A. At point B I read 6. Line segment AB measures 6 cm. Exactly.

Measure a line segment and compare

I have two line segments drawn: AB and CD.

I measure AB: zero on A, at B I read 8 cm.

I measure CD: zero on C, at D I read 5 cm.

I compare: 8 > 5. Line segment AB is longer.

By how much? 8 − 5 = 3 cm. Line segment AB is 3 centimetres longer than CD.

Is it a straight line or a line segment?

In the picture there is a straight line with a dot at the start and a dot at the end.

It has a start and an end → it is a line segment.

The second line has arrows at both ends — a sign that it goes on and on.

It has no start and no end → that is a straight line.

Arrows at the ends are a hint: they say “I go on for ever, I just do not fit on the paper”.

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The most common measuring mistake: you put the edge of the ruler instead of zero. Before zero there is often a bit of extra plastic! Always look for the tick with zero and put that at the start of the line segment.

Practice: now you do it 🎲

Now try it for real. You will see all the questions at once, like on paper. 开始 with ten. If it goes well, add more.

Practise counting differences — it helps when you compare lengths.

Practise the questions

On paper

Clean paper, a sharp pencil and a ruler with a visible zero.

Draw line segments 4 cm, 7 cm and 10 cm long. 姓名 them AB, CD and EF.
Measure all three again to check. Do the lengths match?
Calculate: how many cm longer is EF than AB?
Draw a triangle from line segments: three dots, three joins with a ruler.

Now you 💪

I know how a straight line and a line segment are different.
I put the ruler with zero at the start of the line segment.
I can draw a line segment of an exact given length.

✅

Done when: You draw three line segments of given lengths and the check measurement matches to the centimetre.

What to take from this lesson

A line segment: a straight line from point to point. It can be measured.
A straight line: an endless straight line, with no start and no end.
We mark points with capital letters: line segment AB.
Measure from zero, not from the edge of the ruler.
Comparing lengths is comparing numbers.