In the language of first-order classical logic, a formula of the form

• • ${\text{Q}}_0{x}_0...{\text{Q}}_{n-1}{x}_{n-1}\phi$

where each ${\text{Q}}_i{x}_i$ is either a universal or existential quantifier and $\phi$ is quantifier-free. In classical logic, every formula $\phi$ is logically equivalent to a formula $\psi$ in prenex normal form. For example,

• • $\neg \exists y\left(Ry\wedge \forall x\mathrm{Sxy}\right)$

is classically equivalent to the formula

• • $\forall y\exists x\left(\neg Ry\vee \neg Sxy\right)$.