CMU's Harpy understood a 1,011-word vocabulary of connected speech

Harpy, completed by Bruce Lowerre and Raj Reddy at Carnegie Mellon in 1976 under DARPA’s Speech Understanding Research program, could recognize connected speech over a vocabulary of 1,011 words - sometimes likened to the vocabulary of a three-year-old. It achieved this by compiling its grammar, pronunciation, and acoustic knowledge into a single network and searching it with beam search, pruning all but a narrow band of the most promising interpretations at each step. Beam search proved so useful that it stayed a standard speech-recognition technique for decades.