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More:some 1917 and all later dates - on the back near the 8:00 position
1965 to 1967 - no mint marks
1968 to the present - on the front above the date
Letters could be any of the following, depending on the coin's date:
The mint mark position is on the back, just above the bell's yoke. The letter, if present, will be very small:No mint mark = PhiladelphiaD = DenverS = San Francisco
The US mint did not use mint marks on the 1965 half dollar coin.
Half dollars in 1942 were produced at 3 US Mint facilities; Philadelphia, Pennsylvania {uses no mint mark}, Denver, Colorado {uses the letter "D" as a mint mark, San Francisco, California {uses the letter "S" as a mint mark.
The mint mark for the 1909 Half Eagle is located on the reverse {eagle} side of the coin just to the left of the arrow points.
There were no dollar coins minted between 1936 and 1970 inclusive. Please check your coin again, you probably have a HALF dollar.
$6 to $15,000 depending upon the mint mark and the condition of the coin.
It depends on what date and mintmark it is, post new question
The mint mark position is on the back, just above the bell's yoke. The letter, if present, will be very small:No mint mark = PhiladelphiaD = DenverS = San Francisco
The US mint did not use mint marks on the 1965 half dollar coin.
Half dollars in 1942 were produced at 3 US Mint facilities; Philadelphia, Pennsylvania {uses no mint mark}, Denver, Colorado {uses the letter "D" as a mint mark, San Francisco, California {uses the letter "S" as a mint mark.
US Coins with no mint mark were minted at the US Mint's main facility in Philadelphia, Pennsylvania.
The mint mark for the 1909 Half Eagle is located on the reverse {eagle} side of the coin just to the left of the arrow points.
A mint mark on a half dollar is a letter "P", "D", "CC", "O", or "S", usually much smaller than the other letters on the coin's design. The letters used and their locations vary depending on the coin's date, and up till 1980 US half dollars made in Philadelphia didn't have any mint mark. There's more information at the Related Question.
There were no dollar coins minted between 1936 and 1970 inclusive. Please check your coin again, you probably have a HALF dollar.
For the year 1923 the US only struck 2 types of half dollar coins, a business strike Walking Liberty half with a "S" mint mark and a Monroe Doctrine Centennial Commemorative half dollar also with a "S" mint mark. With out knowing which one you have a assessment is not possible. Post new question with more info.
The US Mint continues to produce half dollar coins today although they are seldom seen in circulation.
The mint mark on a US Trade Dollar is located on the reverse [tails] side of the coin just above the letter "D" in "DOLLAR". No mint mark = Philadelphia S = San Francisco CC = Carson City