The tradition is that a small coin held in the right hand and thrown over the left shoulder means you will return to Rome.
It worked for Doug and I. We were here about 30 years ago. It is estimated that 3000 Euros are thrown in the fountain every day. In 2016, an estimated 1.4 million (US$1.5 million) was thrown into the fountain. The Vatican regularly cleans out the coins and gives them to charity.
(to be continued)