The weight of a syllable depends on the make-up of its rhyme constituent. If the rhyme consists of a long vowel (or diphthong) followed by zero or more consonants (a so-called strong cluster), the syllable is heavy. If the rhyme consists of a short vowel followed by a maximum of one consonant (a weak cluster), then the syllable is light.

Some examples from English:

• sooner 1st syllable heavy because it contains a long vowel. 2nd syllable light
• loudest 1st syllable heavy because it contains a diphthong. 2nd syllable heavy because the short vowel is followed by more than one consonant
• fish a light syllable because the vowel is short and only one consonant follows

The weight of a syllable is important for lexical stress assignment in many languages. Heavy syllables take precedence over light syllables in the assignment of primary lexical stress.