What was the last letter added to the alphabet?

The last letter added to the alphabet is the letter J. Historical records indicate that the letter J was initially a variant of the letter I. In early alphabets, particularly Latin, the letters I and J were interchangeable, and it wasn't until the 16th century that J was distinguished as a separate letter with its own phonetic value. This differentiation made J the last letter to be formally recognized and incorporated into the modern English alphabet.

The other letters listed have long been established in the alphabet and did not undergo this kind of development. For instance, I and K have been present since earlier variations of the alphabet, and Q has consistently been part of the alphabet without the need for later modification or confirmation as a unique letter.