1. In a speech recognition system having an acoustic processor which generates a string of acoustic labels in response to speech input and a decoder which matches words in a vocabulary against generated labels in a string, a method of forming at least one likely sequence of words for a speech input, the method comprising the steps of: (a) generating a string of labels in response to a speech input; (b) selecting words from a vocabulary as possible first words corresponding to labels at the beginning of the string; (c) for a subject selected word, (i) locating a most likely boundary label interval in the string whereat the subject selected words has the highest probability of ending; and (ii) evaluating a respective likelihood of the subject selected word at each label interval of the string up to and including the most likely boundary label interval; (d) repeating step (c) for each selected word as the subject selected word; and (e) classifying a given selected word as extendible if the likelihood at the particular label interval corresponding to the most likely boundary label interval thereof is within a predefined range of the highest likelihood for any selected word at said particular label interval.