According to word maven David Crystal, the letter I was a consonant in the Semitic alphabet and a vowel in Greek; it was adopted by Latin with both consonant and vowel values. The inscription over the cross of Christ illustrates the consonant value: INRI are the initials of the words Iesus Nazarenus, Rex Iudaeorum (Jesus of Nazareth, King of the Jews).
J was developed in the Middle Ages as a fancy calligraphic variant of I, but later came to replace I whenever a consonant value was required. The dot on the lower case i and j was originally a diacritical mark used by medieval scribes, similar to an acute accent, to distinguish the stroke of an i from the identical strokes of m, n, and u when they appeared adjacent to the i.
So you’d better dot you i’s—and cross your t’s, too, while you’re at it.
The Bard of Buffalo Bayou likes to fool around with letters of the alphabet, and he is lucky that he hasn’t been caught at it yet.
If i’s a little dotty,
And t’s a little cross,
It’s because the literati
Forget who is the boss.
For i or t alone,
Counts little, they’ll admit,
But it clearly can be shown
That, together, they are it.