Sometimes c is used for the speed of waves in any material medium, and c 0 for the speed of light in vacuum. This subscripted notation, which is endorsed in official SI literature, has the same form as related electromagnetic constants: namely, μ 0 for the vacuum permeability or magnetic constant, ε 0 for the vacuum permittivity or electric constant, and Z 0 for the impedance of free space. This article uses c exclusively for the speed of light in vacuum.