We quite often find in verse composition that the vowels , , and , have been either lengthened or shortened m.c. End vowels are often subject to these changes, and end vowels in in particular, indeed the lengthening of this vowel m.c. far exceeds all other cases. Occasionally vowels in medial position also undergo change, this being more common than the doubling or simplifying of consonants (which obtains the same result metrically).
The vowels e & o are variable in length, being normally long in open syllables (e.g. upēkhā), and short in closed syllables (e.g. upĕkkhā). Occasionally in verse we find that these vowels must be scanned as short even in open syllables, and, as with the other vowels, this seems to occur particularly when they stand at the end of a word.
Example from Ratanasutta (Khp 6. 10f), where the last syllable in abhabbo must be scanned as light: