All over the world, where ever you go, you will see that people measure length in metre.

This is because metre is the standard unit (SI Unit) for measuring length.

We can measure length in metres by using meter scales and tapes.

If we observe a metre scale, we can notice that it is divided into 100 small divisions by lines.

Each of this division is called a centimetre. $1metre=100centimetre(cm)$ .

Even the centimetre is further divided into 10 small divisions by even smaller lines.

Each of this smaller divisions is called a millimeter.$1centimeter=10millimeter(mm)$.

Metre, centimetre, millimetre are related as shown above.

Let us now understand what is the importance of these smaller units

Suppose we are measuring the length of a desk using a metre scale, and we got the reading as 1.1 metre.

We can comfortably tell everyone that, the length of this table is 1.1 metre.

Now, let's measure the length of a smaller object like an eraser; say we get the reading as 5 cm.

In metres, the reading will be 0.05 m. Now, which one is convenient to use, 0.05 m or 5 cm ??

Obviously, 5 cm is more convenient to use than 0.05 m.

Now, let's measure the thickness of an even smaller object like a coin. In millimetres we will get the readings, say 1.5 mm.

If we measure the thickness of this same coin in cm and m, we will get the reading as 0.15 cm or 0.0015 m.

So, to measure the thickness of a coin, a millimetre is much more suitable than a centimetre or a metre .

Centimetre (cm) and millimetre (mm) are called the $sub−multiples$ of a metre (m) .

Now, if we want to measure a much longer length, such as the distance between two cities - then ? Can we use mm, cm or even m to measure it ?

For example, the distance between Hyderabad and Mysore in metres is around 750,000 m!

Is it really convenient to use metres to measure such long distances ?? No!That is why to cover such larger distances we have another unit called the $kilometre(km)$.

$1km=1000metre$.
Since it is much larger than a m, km is more suitable to measure longer distances.

Clearly, kilometre is a $multiple$ of a metre.

In this unit the distance between Mysore and Hyderabad will be around 750 km, which is much convenient to express.

So depending on the situation and according to our convenience, we use km, m, cm or mm. The ways of conversion between these units are shown above.

Let's take a small REVISION

Sub-multiples of m => cm, mm 1 m = 100 cm ; 1 cm = 10 mm .