If $P$ is a partial order which satisfies the $\kappa$ chain condition and $G$ is a generic subset of $P$ then for any $\kappa<\lambda\in\mathfrak{M}$ , $\lambda$ is also a cardinal in $\mathfrak{M}[G]$ , and if $\operatorname{cf}(\alpha)=\lambda$ in $\mathfrak{M}$ then also $\operatorname{cf}(\alpha)=\lambda$ in $\mathfrak{M}[G]$ .

This theorem is the simplest way to control a notion of forcing, since it means that a notion of forcing does not have an effect above a certain point. Given that any $P$ satisfies the $|P|^+$ chain condition, this means that most forcings leaves all of $\mathfrak{M}$ above a certain point alone. (Although it is possible to get around this limit by forcing with a proper class.)