Parikh's Theorem: CFL's "contain" regular languages?
The Parikh image $\Psi$ of a word is a vector counting the number of each of the letters in the alphabet: for example $\Psi( abbabaaca ) = (5,3,1)$ assuming the alphabet is $\{a,b,c\}$.

The Parikh image of a language is the set of Parikh images of the strings in the language: $\Psi( \{a^nb^nc^n\mid n\ge 0 \}) = \{(n,n,n)\mid n\ge 0 \}$.

The theorem states that the Parikh images of context-free languages are in fact Parikh images of regular languages, as example   $\Psi( \{( ab)^n c^n\mid n\ge 0 \}) = \Psi( (abc)^*) $.

(In a previous edit I had the example  $\Psi( \{a^nb^nc^n\mid n\ge 0 \}) = \Psi( (abc)^*) $. Technically correct, but the reader will note that language is not context-free.)