Background
The U.S. Mint struck the wheat penny from 1909 to 1958 at locations in Denver, San Francisco and Philadelphia. The coin was the first to feature the motto "In God We Trust," which was later adopted for use in many coin and currency issues. The U.S. Mint produced matte proofs for collectors from 1909 to 1916. These hard to find copies can fetch high prices. The mint implemented a new design in 1959 to mark the 150th anniversary of Lincoln's birth. This penny removed the wheat heads and added the Lincoln Memorial in Washington, D.C. to the reverse.
Composition
The wheat penny comprises 95 percent copper and a tin and zinc alloy makes up the other 5 percent of the coin. The penny weighs five gs and is 19 mm in diameter with a plain (non-reeded) edge.
Wartime Penny
The composition of the wheat penny changed in 1943 due to World War II. A shortage of copper led the Treasury Department to use zinc-coated steel for the one-cent piece. The steel pennies were not well received by the public. The previous composition resumed in 1944 and continued through the rest of the wheat penny's history.
Rare Varieties
Less than a half million wheat pennies were minted in San Francisco in 1909 with the designer's initials (V.D.B.) on the reverse. These pennies have sold for more than $600 in good condition and $1,500 in uncirculated condition. Collectors also seek double-die errors from the years 1917, 1936 and 1955. A double-die error occurs when a coin is accidentally struck twice at the mint. The 1955 variety is the hardest to find, and as of 2010 examples that graded as uncirculated sold for $2,000 or more.
Mintmarks
The mintmark on the wheat penny is located below the date on the front. An "S" signifies San Francisco while a "D" stands for Denver. Coins minted in Philadelphia carry no mintmark.
Grades
Numismatists grade wheat pennies with no wear and nice luster as uncirculated (MS-60) or higher. Most examples feature some amount of wear, particularly on the wheat heads on the reverse. If lines of the wheat heads are worn the coin would grade as fine (F-12) or lower.