In inscriptions dating to the early Roman Empire, it is used frequently but inconsistently to transcribe the long vowel /iː/.
In this role it is equivalent to the (also inconsistently-used) apex, which can appear on any long vowel: ⟨á é í ó v́⟩ /aː eː iː oː uː/.
An example would be ⟨fIliI⟩, which is generally spelled fīliī today, using macrons rather than apices to indicate long vowels.
[3] Later on in the late Empire and afterwards, in some forms of New Roman cursive, as well as pre-Carolingian scripts of the Early Middle Ages such as Visigothic or Merovingian, it came to stand for the vowel ⟨i⟩ in word-initial position.
The character exists in Unicode as U+A7FE latin epigraphic letter i longa, ⟨ꟾ⟩, having been suggested in a 2006 proposal.