In linguistics, syllable weight is the concept that syllables pattern together according to the number and/or duration of segments in the rime. In classical Indo-European verse, as developed in Greek, Sanskrit, and Latin, distinctions of syllable weight were fundamental to the meter of the line.
A heavy syllable is a syllable with a branching nucleus or a branching rime, although not all such syllables are heavy in every language. A branching nucleus generally means the syllable has a long vowel or a diphthong; this type of syllable is abbreviated as CVV. A syllable with a branching rime is a closed syllable, that is, one with a coda (one or more consonants at the end of the syllable); this type of syllable is abbreviated CVC. In some languages, both CVV and CVC syllables are heavy, while a syllable with a short vowel as the nucleus and no coda (a CV syllable) is a light syllable. In other languages, only CVV syllables are heavy, while CVC and CV syllables are light. In yet other languages, CVV syllables are heavy and CV syllables are light, while some CVC syllables are heavy (for instance if the coda is a sonorant) and other CVC syllables are light (for instance if the coda is an obstruent). Some languages distinguish a third type, CVVC syllables (with both a branching nucleus and a coda) and/or CVCC syllables (with a coda consisting of two or more consonants) as superheavy syllables.
In moraic theory, heavy syllables are analyzed as containing two morae, light syllables one, and superheavy syllables three.
The distinction between heavy and light syllables plays an important role in the phonology of some languages, especially with regard to the assignment of stress. For instance, in the Sezer stress pattern in Turkish observed in place names, the main stress occurs as an iamb (i.e. penultimate stress) one syllable to the left of the final syllable: (L'L)?. However, when the foot contains a heavy syllable in the first syllable while the second syllable is light, the iamb shifts to a trochee (i.e. antepenultimate stress) because there is a requirement that main stress fall on a heavy syllable whenever possible: ('HL)?, and not *(H'L)?.
In Ancient Greek hexameter poetry and Latin literature, lines followed certain metrical patterns, such as based on arrangements of heavy and light syllables. A heavy syllable was referred to as a longum and a light as a brevis (and in the modern day, reflecting the ancient terms, a longum is often called a "long syllable" and a brevis a "short syllable", potentially creating confusion between syllable length and vowel length).
A syllable was considered heavy if it contained a long vowel or a diphthong (and was therefore "long by nature" -- it would be long no matter what) or if it contained a short vowel that was followed by more than one consonant ("long by position", long by virtue of its relationship to the consonants following).
The first syllable of the first word (arma) is heavy ("long by position") because it contains a short vowel (the A) followed by more than one consonant (R and then M) -- and if not for the consonants coming after it, it would be light. The second syllable is light because it contains a short vowel (an A) followed immediately by only one consonant (the V). The next syllable is light for the same reason. The next syllable, the second syllable of the word virumque, is heavy ("long by position") because it contains a short vowel followed by more than one consonant (the M and then the Q).
But, for example, the first syllable of the word Troiae is heavy ("long by nature") because it contains a diphthong, regardless of the sounds coming after it. Likewise, the first syllable of the second line (the first of the word Italiam) is heavy ("long by nature") because it contains a long vowel, and it will be heavy no matter what sounds come after.
Terming a syllable "long by position" is equivalent to noting that the syllable ends with a consonant (a closed syllable), because Latin and Greek speakers in the classical era pronounced a consonant as part of a preceding syllable only when it was followed by other consonants, due to the rules of Greek and Latin syllabification. In a consonant cluster, one consonant ends the preceding syllable and the rest start the following syllable. For example, Latin syllabifies volat as vo-lat but dignus as dig-nus and monstrum as mon-strum.
A few exceptions to and elaborations of the above rules of heavy and light syllables:
As noted above, the number and order of heavy and light syllables in a line of poetry (together with word breaks) articulated the meter of the line, such as the most famous classical meter, the epic dactylic hexameter.