= System of units
A <system of units> chooses reference <units of measurement> for a set of base <physical quantities> and specifies how other <units of measurement> are derived. In the <International System of Units>, <length>, <mass> and <time> are expressed using the <metre>, <kilogram> and <second>; a <velocity> has derived unit $\mathrm{m\,s^{-1}}$.
A consistent system allows an equation to connect numerical values without hidden unit-conversion factors. It does not change the underlying physical relation: the same <speed> may be written as $1\,\mathrm{m\,s^{-1}}$ or $3.6\,\mathrm{km\,h^{-1}}$. The familiar formulas for <electromagnetism> can nevertheless contain different constants in SI and Gaussian conventions, because the definitions of the electrical quantities differ as well as their numerical scales.<NIST Guide to the SI>[sections 4–6: SI units and use with other units]{ref}
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