Natural speech in all languages is highly rhythmic; it tends to have a regular beat. But it may be rhythmic in different ways, depending on the language. There is a broad division into two kinds of rhythm in language, although some languages fit more clearly into one kind or the other while some languages are more a mixture of the two. (i) Syllabic rhythm, or SYLLABLE-TIMING: in this type of rhythm the tempo depends on the syllable (or on a sub-syllabic unit the mora), so that all syllables tend to be of roughly the same length [...] (ii) Pedalian rhythm, or FOOT-TIMING (commonly called stress-timing): in this type of rhythm the tempo depends on the foot (a unit consisting of one or more syllable), so that all feet tend to be of roughly the same length – which means of course, that the syllables must vary in length, since a foot may consist of varying numbers of syllables.
- Halliday (1985), Pag. 271-272