Jot or tittle is an idiom which means any small thing. Curiously, it is close in origin to the idiom which commands attention to every small detail: dot the is and cross the ts. (That is one place where the urge to use an inappropriate apostrophe is almost irresistible). Jot is a variant of iota, the Greek name for the letter i. Iota is still used alone to mean something small, generally by negation: there is not one iota of evidence …; in exactly the same way it might be said there is not a jot of evidence … The meanings are identical.
The ambivalence between jot and iota is not surprising: until early in the nineteenth century, i and j were facets of the same letter. In the first edition of Johnson’s Dictionary (1755), the entry next after hystericks is I, and it contains a discussion of that letter, followed by its meaning as the first person singular pronoun. The next entry is jabber, followed by other words beginning ja-. After jazel comes ice; after idyl comes jealous, and so on. So it remained in all the editions in Johnson’s lifetime. However the 8th edition, edited by Dr Todd (1818) recognizes that i and j have ceased to be facets of the same thing, and have separated into 2 different letters. Iota and jot are small reminders of the way it was.
So a jot is simply the letter i.[e.a.] A tittle is any diacritic mark in text, such as an accent, a cedilla or a tilde. Nowadays, it refers specifically to the dot above the letter i. So reference to every jot and tittle is a reminder of the importance of dotting the i.
-Julian Burnside
[my reading: jot and tittle is a pleonasm]
