Speech is perceived as a sequence of events in time, and the word rhythm is used to refer to the way these events are distributed in time. Obvious examples of vocal rhythms are chanting as part of games (for example, children calling words while skipping, or football crowds calling their team's name) or in connection with work (e.g. sailor's chants used to synchronise the pulling on an anchor rope). In conversational speech the rhythms are vastly more complicated, but it is clear that the timing of speech is not random. An extreme view (though a quite common one) is that English speech has a rhythm that allows us to divide it up into more or less equal intervals of time called feet, each of which begins with a stressed syllable: this is called the stress-timed rhythm hypothesis. Languages where the length of each syllable remains more or less the same as that of its neighbours whether or not it is stressed are called syllable-timed. Most evidence from the study of real speech suggests that such rhythms only exist in very careful, controlled speaking, but it appears from psychological research that listeners' brains tend to hear timing regularities even where there is little or no physical regularity.
The foot is a unit of rhythm. It has been used for a long time in the study of verse metre, where lines may be divided into sections based on patterns on strong and weak syllables. It is rather more controversial to suggest that normal speech is also structured in terms of regularly repeated patterns of syllables, but this is a claim that has been quite widely accepted for English. The suggested form of the English foot is that each foot consists of one stressed syllable plus any unstressed syllables that follow it; the next foot begins when another stressed syllable is produced. The sentence 'Here is the news at nine o'clock' could be analysed into feet in the following way (stressed syllables underlined, foot divisions marked with vertical lines):
|here is the |news at |nine o |clock
It is claimed that English feet tend to be of equal length, or isochronous, so that in feet consisting of several syllables there has to be compression of the syllables in order to maintain the rhythm. There are many problems with this theory, as one discovers in trying to apply it to natural conversational speech, but the foot has been adopted as a central part of metrical phonology.