 A root datum consists of a quadruple
where 

The elements of formula_5 are called the roots of the root datum, and the elements of formula_7 are called the coroots.If formula_5 does not contain formula_24 for any formula_25, then the root datum is called reduced.If "G" is a reductive algebraic group over an algebraically closed field "K" with a split maximal torus "T" then its root datum is a quadruple 
where 

A connected split reductive algebraic group over "K" is uniquely determined (up to isomorphism) by its root datum, which is always reduced.