CFG rules impose an independence assumption on probabilities that leads to poor modeling of structural dependencies across the parse tree. In a CFD the expansion of a non-terminal is independent of the context, which means that it is not dependent on other nearby non-terminals in the parse tree. Similarly, in a PCFG, the probability of a particular rule is independent of the rest of the tree. Therefore, in a PCFG, we compute the probability of a tree by multiplying the probabilities of each non-terminal expansion. However, this results in poor probability estimates because in English the choice of how a node expands can actually depend on the location in the parse tree of that node. For example, it turns out that NPs which are syntactic subjects are more likely to be pronouns, and NPs which are syntactic objects are more likely not to be. There is no way to represent this contextual difference in the probabilities of a PCFG.