
Since 2007, deceased presidents have been commemorated on $1 coins in the order of their service, according to CoinNews.net.
“Next year marks an end to the series with the commemoration of Richard Nixon, Gerald Ford and Ronald Reagan. They were the 37th, 38th and 40th Presidents of the United States,” states CoinNews.net.
The Presidential $1 Coin Act of 2005 (Public Law 109-145) states that former or current Presidents who are still alive cannot appear on the dollar coins.
Rules about breaking the sequential order of service were less clear.
A legal interpretation was made, and it was decided that the Jimmy Carter, the 39th president, would get passed over to honor Reagan. (Carter is still alive.)