I the ninth letter of the English and Latin alphabet, the tenth
in the Greek and Phoenician, because in these the symbol
Teth (the Greek θ) preceded it. Teth was not included in
the Latin alphabet because that language had no sound
corresponding to the Greek θ, but the symbol was metamorphosed
and utilized as the numeral C = 100, which took this form through
the influence of the initial letter of the Latin centum. The name
of I in the Phoenician alphabet was Yōd. Though in form it
seems the simplest of letters it was originally much more complex.
In Phoenician it takes the form 
, which is found also in the
earliest Syriac and Palestinian inscriptions with little modification.
Ultimately in Hebrew it became reduced to a very small
symbol, whence comes its use as a term of contempt for things
of no importance as in “not one jot or tittle” (Matthew v. 18).
The name passed from Phoenician to Greek, and thence to the
Latin of the vulgate as iōta, and from the Latin the English
word is derived. Amongst the Greeks of Asia it appears only
as the simple upright I, but in some of the oldest alphabets
elsewhere, as Crete, Thera, Attica, Achaia and its colonies in
lower Italy, it takes the form 
or S, while at Corinth and
Corcyra it appears first in a form closely resembling the later
Greek sigma Σ. It had originally no cross-stroke at top and
bottom. I being not i but z. The Phoenician alphabet having
no vowel symbols, the value of yōd was that of the English y.
In Greek, where the consonant sound had disappeared or been
converted into h, I is regularly used as a vowel. Occasionally,
as in Pamphylian, it is used dialectically as a glide between i and
another vowel, as in the proper name Δαμάτριιυς.. In Latin I
was used alike for both vowel and consonant, as in iugum (yoke).
The sound represented by it was approximately that still assigned
to i on the continent. Neither Greek nor Latin made any
distinction in writing between short and long i, though in the
Latin of the Empire the long sound was occasionally represented
by a longer form of the symbol I. The dot over the i begins in
the 5th or 6th century A.D. In pronunciation the English
short i is a more open sound than that of most languages, and
does not correspond to the Greek and Latin sound. Nor are
the English short and long i of the same quality. The short i
in Sweet’s terminology is a high-front-wide vowel, the long i,
in English often spelt ee in words like seed, is diphthonged,
beginning like the short vowel but becoming higher as it proceeds.
The Latin short i, however, in final syllables was open and
ultimately became e, e.g. in the neuter of i-stems as utile from
utili-s. Medially both the short and the long sounds are very
common in syllables which were originally unaccented, because
in such positions many other sounds passed into i: officio but
facio, redimo but emo, quidlibet but lubet (libet is later); collīdo
but laedo, fīdo from an older feido, istis (dative plural) from an
earlier istois.
(P. Gi.)