Ron Guth: In 1965, because of rising costs, the Mint began reducing the amount of silver in the coins it produced. Silver was eliminated completely from the Dimes and Quarter Dollars, in favor of a new composition consisting of a copper core sandwiched between outer layers of copper-nickel. For the Half Dollar, the Mint reduced the amount of silver from 90% to 40%, but retained much of the whitish, silvery appearance of former years by placing most of the silver in the outer layers. The new Half Dollars became known as "Clad" Half Dollars, but this term is also used as a nickname for 1965 and later Dimes, Quarter Dollars, and One Dollars (clearly, the term refers more to the layered structure of the coins than to their metal content).
No Proof Half Dollars were struck in 1965, 1966, or 1967. Instead, the Mint made Special Mint Sets, which contained coins with a special finish that was better than that seen on a circulation strike, but which did not rise to the quality of a Proof coin. These were produced by overpolishing the dies.
This series ended in 1970, after which the composition was matched to the copper-nickel clad alloy of the Dimes and Quarter Dollars. No 1970 Half Dollars were produced for circulation; they are only available in Mint or Proof Sets.
No major varieties exist in this series.