The Old Latin diphthong "ei" evolves in stages: "ei" > "ẹ̄" > "ī". The intermediate sound "ẹ̄" was simply written "e" but must have been distinct from the normal long vowel "ē" because "ẹ̄" subsequently merged with "ī" while "ē" did not. It is generally thought that "ẹ̄" was a higher sound than "e" (e.g. perhaps vs. during the time when both sounds existed). Even after the original vowel had merged with "ī", the old spelling "ei" continued to be used for a while, with the result that "ei" came to stand for "ī" and began to be used in the spelling of original occurrences of "ī" that did not evolve from "ei" (e.g. in the genitive singular "-ī", which is always spelled "-i" in the oldest inscriptions but later on can be spelled either "-i" or "-ei").