Starting in 1965 the Mint removed all silver content from Roosevelt Dimes. Therefore, this was the second year in which they struck Clad Dimes. The Mint struck over a billion 1966 dimes, so they are very common in circulated grades and up to MS64 grade. MS65 grade coins are probably from rolls or coins that were pulled from circulation very early. In MS66 condition they are a lot harder to find most of them probably pulled from rolls but can still be purchased very easily raw or graded inexpensively as there are thousands of them in existence. In MS67 grade much more difficult to find. In MS68 condition they are very scarce with less than a dozen known and none being finer.