When a word ending in a vowel meets one beginning with a vowel, the first is swallowed — ELISION — and the two syllables count as one for the metre. It’s how a line that looks too long scans perfectly. Count the written syllables, subtract one for every vowel-collision, and you get the syllables the ear actually keeps. Slide the collisions and watch the count fall to fit.
Elision removes a word-final vowel (and, in Greek/Latin, sometimes a final -m or -s context) before a word beginning with a vowel, merging the two into one metrical syllable. So the SCANNED syllable count = written syllables − number of vowel-collisions at word boundaries. It reconciles a written line with a fixed metrical length (e.g. a hexameter’s slots) — the poet writes more and the metre hears fewer. A fail-loud self-check throws unless each collision reduces the scanned count by exactly one. ◆ the counting rule is exact, node-verified.
The collision-counting is exact arithmetic; whether a specific junction elides (hiatus can be permitted for effect) is a matter of the poet’s practice and the editor’s ear — the rule is clean, its application has exceptions.