Dimensional quantities such as length, mass and charge, i.e., numbers combined with a conventional unit, are essential components of theories in the sciences, especially physics, chemistry and biology. Do they represent a world with absolute physical magnitudes, or are they merely magnitude ratios in disguise? Would we notice a difference if all the distances or charges in the world suddenly doubled? These central questions of this Element are illustrated by imagining how one would convey the meaning of a kilogram to aliens if one were only allowed to communicate via Morse code.