History and usage
See also: Turkish dotted and dotless I
A number of alphabets use dotted and dottless I, both upper and lower case.
In the modern Turkish alphabet, the absence or presence of a tittle distinguishes two different letters representing two different phonemes: the letter "I" / "ı", with the absence of a tittle also on the lower case letter, represents the close back unrounded vowel [ɯ], while "İ" / "i", with the inclusion of a tittle even on the capital letter, represents the close front unrounded vowel [i].
In the latest Latin based Kazakh alphabet, there are also dotted and dotless letter i and I for different sounds.
It should be noticed that when an I or J receive a accent mark, the tittle is replaced.

Dotless and Dotted I

It is thought that the phrase "to a T" is derived from this word.
The phrase "to dot ones i's and cross one's t's" is used to mean either to be thorough or to be pedantic. See also

Dictionary.com — Tittle
Not One "Jot or Tittle"