We describe a parsing system based upon a language model for English that is, in turn, based upon assigning probabilities to possible parses for a sentence. This model is used in a parsing system by nding the parse for the sentence with the highest probability. This system outperforms previous schemes. As this is the third in a series of parsers by di erent authors that are similar enough to invite detailed comparisons but di erent enough to give rise to di erent levels of performance, we also report on some experiments designed to identify what aspects of these systems best explain their relative performance.