Almost every United States coinage series has certain dates that are difficult to obtain for one reason or another. Some dates have exceptionally low mintages. Coins of these dates are key coins that are fundamentally rare. That is, they are rare in all conditions because only a limited number was struck. Other coins are rare in uncirculated or higher uncirculated grades. These coins are called condition rarities and are often key coins as well. Then there are some coins that are available in all conditions, but since the series is so popular, the scarcer dates become key coins even though in absolute numbers they are not rare. Finally, some coins are actually mint errors that become rare because of collector demand. This category includes doubled dies and re-punched dates and mintmarks.
Some examples can be illustrative. The 1794 Silver Dollar had a mintage of 1,758. No matter how many millions of collectors there are in the world, only that number of collectors can obtain an example of this coin. In fact, the actual number is much less because this is only a mintage number and does not account for the actual survival rate for this issue. In addition, many examples are impounded in museums and national collections making them unobtainable by the general public, even for a price. Certainly the 1794 dollar is a key date coin. Continuing with dollars, an 1872-S Seated Liberty dollar can be obtained in circulated condition for three to five hundred dollars. In mint state 65 it lists for $131,250, clearly a condition rarity and a key coin in higher mint state grades. The High Relief, Roman Numeral Double Eagles of 1907 had a mintage of 12,367, not a tiny mintage by any means. Yet because of the number of collectors who desire an example, the coin is always in demand in all conditions and especially in mint state. Finally, there are thousands of different doubled dies in many series, but there is none more famous than the Lincoln 1955 Doubled Die Obverse coin. This coin has become a key date although many other doubled dies, re-punched dates and mintmarks go unnoticed except by specialists.
The following list of key date coins does not include proofs unless they are "proof only" for the date. For example, the 1895 Morgan Dollar is a "proof only" coin since no business strikes were released for that year. Collectors would have to obtain a proof coin to have a complete date set that included 1895.
Half-Cents and Cents
Half Cent 1793, 1796 With Pole, 1796 No Pole, 1797 Gripped Edge, 1802/0 Reverse of 1800, 1805 Small 5 Stems,1808/7, 1811 Close Date
Large Cent 1793 all types, 1794 Starred Reverse, 1795 Jefferson Head Plain Edge, 1796 LIHERTY, 1803 Large Date Small Fraction, 1804, 1807/6 Small 7, 1823
Flying Eagle Cent 1856
Indian Cent 1877, 1888/7
Lincoln Cent 1909-S VDB, 1914-D, 1922-D Missing D Strong Reverse, 1955 Doubled Die Obverse
Two and Three Cents
Two Cent 1864 Small Motto, 1867 Doubled Die Obverse
Three Cent Silver 1855
Three Cent Nickel 1883, 1884, 1885
Nickels
Shield Nickel 1880, 1883/2
Liberty Nickel 1885, 1912-S
Buffalo Nickel 1914/3, 1916 Doubled Die Obverse, 1918/7-D, 1926-S, 1935 Doubled Die Reverse, 1937-D 3 Legs
Jefferson Nickel 1942-D/Horizontal D
Half-Dimes and Dimes
Half Dime 1797 13 Stars, 1802, 1803 Small 8, 1805, 1838-O, 1840-O, 1844-O, 1846, 1853-O, 1870-S
Draped Bust Dime 1804 14 Stars Reverse
Capped Bust Dime 1822
Liberty Seated Dime 1846, 1871-CC, 1872-CC, 1873-CC, 1874-CC
Barber Dime 1894-S, 1895-O
Mercury Dime 1916-D, 1919-D, 1919-S, 1942/1, 1942/41-D
Roosevelt Dime 1968-S No S, 1975-S No S
Quarters
Twenty Cent 1875-CC, 1876-CC
Draped Bust Quarter 1796, 1804
Capped Bust Quarter 1822 25/50, 1823/2, 1827/3
Liberty Seated Quarter 1840-O Drapery at Elbow Large O, 1842-O Small Date, 1855-O, 1856-S, 1864-S, 1867-S, 1871-CC, 1872-CC, 1873-CC, 1874-CC
Barber Quarter 1896-S, 1901-S, 1913-S
Liberty Standing Quarter 1916, 1918/7-S, 1927-S
Washington Quarter 1932-D, 1937 Doubled Die Obverse, 1950-D/S
Half Dollars
Flowing Hair Half Dollar 1794, 1795 Small Head, 1795 Silver Plug
Draped Bust Half Dollar 1796, 1797, 1802, 1806 Knobbed 6 No Stem
Capped Bust Half Dollar 1807 Bearded Goddess, 1812/1 Large 8, 1815/2, 1817/4, 1830 Large Letters, 1838-O
Liberty Seated Half Dollar 1842-O Small Date, 1847/6, 1871-CC, 1872-CC, 1873 Open 3, 1873-CC, 1874-CC, 1878-S
Barber Half Dollar 1892-O Micro O, 1904-S
Walking Liberty Half Dollar 1919-D, 1921-D, 1921-S
Franklin Half Dollar 1953-S Full Strike
Dollars
Flowing Hair Dollar 1794, 1795 Silver Plug
Draped Bust Dollar 1796, 1797, 1798, 1800 AMERICAI, 1801, 1802 Wide Date
Liberty Seated Dollar 1850-O, 1851, 1852, 1858, 1859-S, 1870-CC, 1870-S, 1871-CC, 1872-CC, 1873-CC
Trade Dollar 1873-CC, 1876-CC Double Die Reverse, 1878-CC, 1884 Proof
Morgan Dollar 1879-CC, 1889-CC, 1892-S, 1893-S, 1895 Proof, 1895-O
Peace Dollar 1924-S, 1925-S, 1927-S, 1928-S, 1934-S
Gold Coins
Gold Dollar 1849-C, 1850-D, 1852-D, 1854-D, 1855-C, 1855-D, 1856-D, 1857-C, 1857-D, 1858-D, 1860-D, 1861-D, 1875
Draped Bust $2.5 1796, 1797, 1798, 1804 13 Stars on Reverse, 1806/5
Capped Bust $2.5 1808, 1826, 1834
Classic Head $2.5 1838-C, 1839-C, 1839-D, 1839-O
Liberty Head $2.5 1840-D, 1841, 1841-D, 1842-C, 1842-D1843-C Small Date, 1845-O1848 CAL, 1849-C, 1854-D, 1855-C, 1855-D, 1856-D, 1859-D, 1862-S, 1864, 1865, 1875
Indian $2.5 1911-D
Three Dollar 1854-D, 1854-O, 1855-S, 1857-S, 1860-S, 1873, 1875, 1876, 1877
$4 Stella 1879, 1880
Draped Bust $5 1797, 1798, 1795, 1797, 1798, 1799
Capped Bust $5 1810, 1815, 1819, 1820, 1821, 1824, 1825/4, 1828/7, 1829, 1830, 1831, 1832, 1833, 1834
Classic Head $5 1834 Crosslet 4, 1838-C, 1838-D
Liberty Head $5 1839-D, 1840-C, 1842-C Small Date, 1842-D Large Date Large Letters, 1847-O, 1850-D, 1861-C, 1861-D, 1861-S, 1862-S, 1863-S, 1870-CC, 1871-CC, 1873-CC, 1874-CC, 1875, 1875-CC, 1875-S, 1876-CC, 1877-CC, 1878-CC, 1887 Proof
Indian $5 1909-O, 1929
Draped Bust $10 1795 9 Leaves, 1798/7 9x4 Stars, 1798/7 7x6 Stars
Liberty Head $10 1838, 1839, 1841-O, 1842-O, 1849-O, 1852-O, 1855-O, 1855-S, 1857-O, 1858, 1858-S, 1859-O, 1859-S, 1860-S, 1861-S, 1862-S, 1863, 1863-S, 1864-S, 1865, 1865-S, 1865-S 865/Inverted 186, 1866-S No Motto, 1866, 1866-S Motto, 1867-S, 1869, 1869-S, 1870-CC, 1870-S, 1871-CC, 1871-S, 1872-CC, 1873, 1873-CC, 1874-CC, 1874-S, 1875, 1875-CC, 1876, 1876-CC, 1877-CC, 1878-CC, 1879-CC, 1879-O, 1882-CC, 1883-O
Indian $10 1907 Wire Edge, 1907 Rolled Edge, 1920-S, 1930-S
Liberty Head $20 1850-O, 1851-O, 1852-O, 18853/2, 1853-O, 1854 Large Date, 1854-O, 1855-O, 1856-O, 1857-O, 1858-O, 1859, 1859-O, 1860-O, 1861-O, 1861-S Paquet Reverse, 1666-S, 1866-S, 1870-CC, 1871-CC, 1872-CC, 1873-CC, 1878-CC, 1879-CC, 1879-O, 1881, 1882, 1883 Proof, 1884 Proof, 1885, 1885-CC, 1886, 1887 Proof, 1891, 1891-CC
St. Gaudens $20 1907 High Relief, 1920-S, 1921, 1927-D, 1927-S, 1930-S, 1931, 1931-D, 1932
Commemoratives
Early Commemoratives 1900 Lafayette Dollar, 1904 & 1905 Lewis & Clark Dollar, 1915-S Panama-Pacific $2.5, 1915-S Panama-Pacific Fifty Dollars Round, 1915-S Panama-Pacific Fifty Dollars Octagonal, 1922 Grant Half Dollar Star, 1928 Hawaii Half Dollar.