We propose a new type of regulation on the derivation of a context-free grammar: the productions used for passing from a level of a derivation tree to the next level should have the right-hand members of the same length. We prove that such length synchronized context-free grammars characterize the family of ET0L languages, and therefore are equivalent with the synchronization context-free grammars of H. Jiirgensen and K. Salomaa. In this way, a conjecture of H. J{\"{u}}rgensen and K. Salomaa is disproved, about a language which was conjectured not to be generated by a synchronization context-free grammar.

Journal of Automata, Languages and Combinatorics, Volume 9, Number 4, 2004, 457-464