J

The tenth letter of the Latin alphabet, J was originally only a capital letter, therefore, some people still write their names as Jsabel, Jnes instead of Isabel, Ines in the German-speaking world, and in Italy, in pre-modern use one also sometimes encounters J as a capital of I.

A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z

The Humanistic scholar Pierre de la Ramée (d. 1572) was the first to make a distinction between I and J. Originally, both I and J were pronounced as [i], [i:], and [j]; but Romance languages developed new sounds (from former [j] and [g]) that came to be represented as I and J; therefore, English J has a sound quite different from I.

In other Germanic languages J stands for /j/.

In modern standard Italian only foreign or Latin words have J. Until the 19th century, J was used instead of I in diphthongs, as a replacement for final -ii, or in vowels groups (as in Savoja); this rule was quite strict for official writing. J is also used for rendering words in dialect, where it stands for /j/, e.g. Romanesque ajo for standard aglio (garlic).

In Spanish J stands for /x/ (that in some cases developed from the /dZ/ sound, i.e. the same sound that English still has). In French former /dZ/ is now pronounced as /Z/ (as in English MEASURE).

In Turkish, Azeri and Tatar J is always prounced [zh]. (see SAMPA for meaning of all those phonetic symbols).

Juliet represents the letter J in the NATO phonetic alphabet.

J is also:

As j, the symbol for the imaginary unit (the square root of -1) in physics.
As j, one of the three imaginary units of quaternions.
The symbol for the SI unit of energy, the joule.