Reference

The letter, and where to look for it

A mint mark is one or two small letters saying which facility struck the coin. On half dollars it has moved twice, disappeared entirely for three years, and it is the difference between a common date and a scarce one.

The marks

Mark	Mint	Notes
none or P	Philadelphia	The original mint. Half dollars carried no mark at all until 1980, when a P was added.
D	Denver	Struck half dollars from 1906 onward. Still in production today.
S	San Francisco	From 1855. Now strikes proofs only, including the silver proofs sold in sets.
O	New Orleans	1838 to 1909. The mint closed to coinage in 1909 and the mark never returned.
CC	Carson City	1870 to 1878 only, on Seated Liberty halves. Short-lived and collected hard.
W	West Point	Used on the 2014 gold Kennedy anniversary issue rather than on any circulating coin.

Where it sits, by series

Flowing Hair

1794-1795

None. Philadelphia was the only mint.

Draped Bust

1796-1807

None. Philadelphia was the only mint.

Capped Bust

1807-1839

None until 1838. New Orleans struck a tiny 1838-O issue.

Seated Liberty

1839-1891

Reverse, below the eagle.

Barber

1892-1915

Reverse, below the eagle.

Walking Liberty

1916-1947

Obverse below the motto in 1916 and part of 1917, then reverse at lower left for the rest of the series.

Franklin

1948-1963

Reverse, above the beam of the Liberty Bell.

Kennedy

1964 to today

Reverse in 1964, none from 1965 to 1967, then obverse below the neck from 1968 onward.

The three years with no mark

Coins dated 1965, 1966 and 1967 carry no mint mark at all, regardless of where they were struck. The country was in a coin shortage, the Mint blamed collectors for pulling coins out of circulation, and removing the marks was a deliberate attempt to make the coins less interesting to collect. Marks returned in 1968, and on the Kennedy half they came back in a new place: the obverse, below the neck, rather than the reverse.

Why it matters

The same date from a different mint can have been struck in wildly different numbers. A common Philadelphia issue and a scarce San Francisco one look identical apart from one letter.
On Walking Liberty halves the mark moved from obverse to reverse during 1917, so both exist for that date and they are not equally common.
Mint marks were punched into the dies by hand until the 1980s, which is why repunched and misplaced marks exist and are collected as varieties.
A mint mark says nothing about silver content. Composition was set by date, not by facility. Check the date for that.

Mint mark positions are Mint specifications. This site publishes no mintage figures, so it will not tell you how many of a given date and mint were struck; here is why.