When you take a measurement, there is a true value you are trying to measure. We use measuring tools to try our best to find what that true value is.

As an example, the magnet below has a length, and that length has a true length. We can then use a ruler to try and find the true length, but the measurement we take is then called the measured value.

Assuming a good measurement has been taken, the measured value should be very close to the true value.

This means that you need to distinguish between the true length, and your measured length.

Just below is another example of a student trying to find the true width of a toy van with a ruler:

The final image below shows another example of a student trying to find the true volume of a liquid with a measuring cylinder. The measuring cylinder is the measuring tool here.