Languages can be classified as stress-timed and syllable-timed. The idea was first expressed by Kenneth Pike in 1945. In stress-timed languages, the amount of time used for a stressed syllable is greater than for an unstressed syllable. With syllable-timed languages, however, each syllable is spoken in approximately the same length of time.
Examples of stress-timed languages include Swedish, English, German, Russian, Thai and Arabic. Many stress-timed languages such as English have reduced vowels in unstressed syllables. Arabic, a language with no reduced vowels, is an exception to this rule.
Syllable-timed languages include French, Italian, Spanish, Cantonese, Icelandic and Welsh. Most syllable-timed languages lack reduced vowels. French is an exception.
The concept of stress-timed and syllable-timed languages has many supporters. The reality, though, is that the languages of the world don't fit so precisely into these two categories. It may in fact be better to state that all languages display characteristics of both syllable-timed and stress-timed languages.