 Upward Lowenheim-Skolem theorem

If an $L$-theory has an infinite model, then it has models of arbitrarily large cardinality. For a cardinal $\kappa$, add constants $c_{\alpha }$ for $\alpha <\kappa$ and all inequalities $c_{\alpha }\ne c_{\beta }$. Every finite fragment is satisfiable in the given infinite model, so compactness gives a model with at least $\kappa$ elements. Applying the downward theorem produces a model of exactly $\kappa$ when $\kappa \ge |L|+{\aleph }_0$.