In Christian art, often the letters IHS or JHS are used to represent the name of Jesus.
Is there a difference between IHS and JHS?
Starting in the 3rd century, Christians shortened the name of Jesus by only writing the first three letters of his name in Greek, ΙΗΣ (from his full name ΙΗΣΟΥΣ). The Greek letter Σ (sigma), is written in the Latin alphabet as an “S,” resulting in the monogram being commonly represented as ΙΗS.
One of the main reasons this monogram has the letter "I" in it is because both Greek and Latin did not use the letter "J" for many centuries.
According to Latin scholar John Byron Kuhner, in an article for Inside the Vatican, this issue came-up when St. John Paul II wrote his name in Latin using a "J." At the time, Fr. Reginald Foster was at the Vatican and pointed out the error.
By the 16th century the letter "J" became used in many Latin inscriptions, including the "JHS."
