In Classical Latin, the letter ⟨I⟩ was used for both the /i/ sound (a vowel) and the /j/ sound (the consonant version of that vowel).

The letter ⟨J⟩ developed as a variant of ⟨I⟩. At first the two were used interchangeably as versions of the same letter, but by the 1600s they had become distinct letters representing distinct sounds.

The dot over the lowercase ⟨i⟩ (called a tittle) wasn’t added until later, to help distinguish it from all the other short vertical strokes in letters like ⟨m⟩, ⟨n⟩, and ⟨u⟩ (often called the minim problem).

Here’s an image of the Latin phrase “mimi numinum niuium minimi munium nimium uini muniminum imminui uiui minimum uolunt” in Gothic handwriting. (Note that some ⟨u⟩’s are consonants and some are vowels.)

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