Although the phenomenon itself has been known since ancient times, the phrase is said[1] to have been invented by the classical scholar Paul Maas.
[2] Brevis in longo is possible in various classical metres that require a long syllable at the end of a line, including dactylic hexameters and iambic trimeters.
[1] A similar phenomenon is found in other languages whose poetic metres are quantitative, such as Arabic, Persian, and Sanskrit.
Brevis in longo is distinct from the metrical element anceps, which is a position in a line which can be filled by either a long or a short syllable.
[1] Ancient writers on Latin and Greek metre noted that it was logical for a short syllable at the end of a line to be taken as long, since the pause helped to make up the length.
[13] Thus a normally short vowel can stand in final position in metres such as the ṭawīl, where the repeating pattern of the feet leads one to expect a long syllable at the end: Exactly as with Greek, a short penultimate becomes long when a line is made catalectic.
Thus the most commonly used Arabic metre, the ṭawīl, has normal and catalectic forms as follows:[14] In classical Persian, just as in Latin and Greek, poetic metres are quantitative, except that in addition to long and short syllables, Persian also has "overlong" syllables which are equivalent in length to long + short.
The lengthening of a penultimate short syllable when a line is made catalectic is found in Persian too; for example, if a metre in choriambic (– u u –) rhythm is shortened, the resulting ending is a cretic (– u –).