when measuring the length it is very important to use the correct tools and units to attain the greatest accuracy. For example to measure a length of a book we can take length unit as cm, and we can measure by a scale . But to measure the thickness of a paper we have to take the unit of micrometer , and we have to use screw gauge to measure the thickness of the paper. We can't not measure the thickness of a paper by a meter scale. And it will not be helpful if we use unit as meter.