1.2 Units

When exploring different scales, you will inevitably encounter a number of different units.

There is an International System of Units that gives a preferred unit (known as an SI unit) for each type of measurement. For time, this is seconds (abbreviated to s) and for distance this is metres (m). These are both known as base units.

There are seven base units in the SI system and every other SI unit can be expressed as a combination of these. As you will see, the main axes of the size–time explorer uses SI units, but you will come across other units that are also used for time and length. Note that when the symbols for units are used, do not make them plural. For example, write ‘4 cm’, not ‘4 cms’.