The name Jon functions as an alternative spelling of John, but more frequently as a short form of Jonathan. In the Hebrew Bible, Jonathan was the son of King Saul and a close friend of David. In the English-speaking world, the name wasn’t common until the Protestant Reformation. In the US the popularity of Jonathan started growing in the 1930s and currently it is one of the most popular names given to boys. Jon, on the other hand, was the most popular in the 1960s, since then its popularity has declined.