As we have seen already with percentages, relative length units are relative to other length values. The most commonly used unit is the em unit. This is a font relative unit, as it’s calculated based on a parent element’s font size. So, whenever we use an em unit as the value for an elements CSS property, that em value is equal to the font size value of the parent element.
So, for example, the default font size for browsers is 16 pixels. If we set our page’s body font size to 1em, then this is equivalent to setting the font size to 16 pixels. This means that by default, 1em is equivalent to 16 pixels, 2ems is equivalent to 32 pixels and so on and so forth.