17. The system of claim 15 , wherein the scores of the maximum entropy language model comprise, for each context in a set of different contexts that each comprise a different sequence of one or more words, scores for: multiple different n-gram features that each correspond to the occurrence of a respective word with the context, and a backoff feature that corresponds to the occurrence of any of multiple words with the context.
15. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: receiving, by the one or more computers, data indicating a candidate transcription for an utterance and a particular context for the utterance; obtaining, by the one or more computers, a maximum entropy language model that includes (i) scores for one or more n-gram features that each correspond to a respective n-gram and (ii) scores for one or more backoff features that each correspond to a set of n-grams for which there are no corresponding n-gram features in the maximum entropy language model; determining, by the one or more computers, based on the candidate transcription and the particular context, a feature value for (i) each of the one or more n-gram features of the maximum entropy language model and (ii) each of the one or more backoff features of the maximum entropy language model; inputting, by the one or more computers, the feature values for the n-gram features and the feature values for the backoff features to the maximum entropy language model; and receiving, by the one or more computers, from the maximum entropy language model, an output indicative of a likelihood of occurrence of the candidate transcription; selecting, by the one or more computers, based on the output of the maximum entropy language model, a transcription for the utterance from among a plurality of candidate transcriptions; and providing, by the one or more computers, the selected transcription to a client device.