Add a new page:
Any physical quantity can be expressed by using as fundamental unit either energy, mass, length or time.
\begin{align} ct&\approx \ell\Longrightarrow {\rm time}\approx{\rm length},\\ E&\approx mc^2\Longrightarrow {\rm energy}\approx{\rm mass},\\ E&\approx pv\Longrightarrow {\rm energy}\approx{\rm momentum},\\ Et&\approx \hbar\Longrightarrow {\rm energy}\approx{\rm (time)^{-1}}\approx {\rm (length)^{-1}}. \end{align}