Binarization of n-ary rules is critical for the efficiency of syntactic machine translation decoding. Because the target side of a rule will generally reorder the source side, it is complex (and sometimes impossible) to find synchronous rule binarizations. However, we show that synchronous binarizations are not necessary in a two-stage decoder. Instead, the grammar can be binarized one way for the parsing stage, then rebinarized in a different way for the reranking stage. Each individual binarization considers only one monolingual projection of the grammar, entirely avoiding the constraints of synchronous binarization and allowing binarizations that are separately optimized for.
