The dot above a lowercase i or j has a name: a tittle.
The word comes from the Latin titulus, meaning a small mark or title. The dot was added in medieval manuscripts to make the letter i stand out from the strokes of letters like m, n and u around it.
Why is this true?
The tittle is where the phrase to a T probably comes from, via the older expression to a tittle, meaning to the smallest detail. In Turkish, dotted and dotless i are two different letters.
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Language facts show how much of everyday speech is historical accident. Words that once meant the opposite, letters that vanished from the alphabet, punctuation with names almost nobody uses. Each entry explains the change that produced the oddity — a sound shift, a printing convention, a borrowing from another language — so it works as a genuine explanation and not just as a piece of trivia.