Relative Lengths

specify a length relative to another length. Style sheets that use relative units can more easily scale from one output environment to another
Unit Relative To Example
em font size of the elment p { line-height: 1.5em }
ex x-height of the elemnt's font
ch width of the "0" (zero) glyph in the elemnt's font
rem font size of the root element
vw 1% of viewport's width
vh 1% of viewport's height
vmin 1% of viewport's smaller dimension
vmax 1% of viewport's larger dimension