Ever wonder why I and j are side by side in the alphabet when they look so similar? Well, they actually started out as the same letter.
The letter j's swash was just an embellishment on I. The two would be used interchangeably. This explains the 'j' in the word hallelujah.
With the introduction of the Roman numeric system, j was also used to punctuate a series of one's, like xiij for the number 13.
It seemed that 'j' would forever be doomed to life as a fancy 'I' until 1524 when an Italian Renaissance grammarian Gian Giorgio Trissino distinguished the two sounds.
He introduced the soft 'j' sound, as in "jam." For this, Trissino was named father of the letter "j". Yet ironically, his name contains 5 I's and no j's.