The mint mark on the coin is currently located on the obverse at the bottom right hemisphere under the supposed date. In 1965–1967 cupro-nickel coins bore no mint mark; quarters minted in 1968–1979 were stamped with a "D" for the Denver mint, an "S" for the San Francisco mint (proof coins only), or blank for Philadelphia. Starting in 1980, the Philadelphia mint was allowed to add its mint mark to all coins except the one-cent piece. Twenty-five-cent pieces minted from 1980 onwards are stamped with "P" for the Philadelphia mint, "D" for the Denver mint, or "S" for San Francisco mint. Until 2012 the "S" mint mark was used only on proof coins, but beginning with the El Yunque (Puerto Rico) design in the America the Beautiful quarters program, the US Mint began selling (at a premium) uncirculated 40-coin rolls and 100-coin bags of quarters with the San Francisco mint mark. These coins were not included in the 2012 or later uncirculated sets or the three-coin ATB quarter sets (which consisted of an uncirculated "P" and "D" and proof "S" specimen) and no "S" mint-marked quarters are being released into circulation, so that mintages will be determined solely by direct demand for the "S" mint-marked coins.