In classical Indo-European verse, as developed in Greek, Sanskrit, and Latin, distinctions of syllable weight were fundamental to the meter of the line.
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.
In Ancient Greek hexameter poetry and Latin literature, lines followed certain metrical patterns, such as based on arrangements of heavy and light syllables.
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.
(The word "Italiam" is a special case, in that poets treat it as having a long-by-nature first syllable which it actually has not, in order to make it fit somehow.)
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.