That would depend on the country of origin, the coin and the mint concerned.
Not all coins have mintmarks. They are usually used when the same coin is minted at two or more different mints.
Coins minted at foreign mints will nearly always have a mintmark.
Mintmarks are typically found on coins, near the design or date. They indicate which mint facility produced the coin. On United States coins, mintmarks can be located on the reverse side near the bottom, above or below the denomination.
If the coin does have a mintmark it would be under the "DO" in dollar on the back of the coin.
underneath the year
There is no mintmark on the 1921 British Penny, they were all minted at the Royal Mint London.
If you refer to the British Five Pound (Crown) coin, they do not have a mintmark. They were all minted at the Royal Mint in Wales.
If it is a "s" quarter it will always have the S mintmark. You can find this mintmark usually on the back of the coin. The exact location will depend on the type of quarter.
Any mintmark will be found on the reverse of the coin under the wreath above the space between the D and O in "DOLLAR". Keep in mind that your coin could have no mintmark, in which case it would be minted in Philadelphia. If your coin has a mintmark, it will ether be a "CC", a "S" or an "O".
British Edward VII (1902-1910) Sovereigns and Half-Sovereigns were minted at the Royal Mint London, Ottawa, Melbourne, Perth and Sydney Mints. The coins minted in London have no mintmark. The Sovereign and Half-Sovereign coins minted elsewhere have a mintmark "on the ground" below the horse and above the date. The coins minted in Ottawa have a mintmark "C". The coins minted in Melbourne have a mintmark "M". The coins minted in Perth have a mintmark "P". The coins minted in Sydney have a mintmark "S". The letters BP to the right of the date is the designers initials (Benedetto Pistrucci) who designed the St George and the Dragon scene.
The letter "D" under the date of a penny is the mintmark which indicates where the coin was minted. There are 4 operating mints in the USA. One , represented by the mintmark "D" is in Denver, Colorado. Another, represented by the mintmark "S" is in San Francisco, California. Another, represented by the mintmark "W" is in West Point, New York. The 4th, represented by the mintmark "P" is in Philadelphia, Pennsylvania. You will find these mintmarks on all coins struck in the USA today with the exception of the pennies which are produced at the Philadelphia mint. You will find no pennies with a mintmark "P". You will also find no coin minted in 1965, 1966 and 1967 with a mintmark. This is because during those years, to discourage coin collecting which some blamed for a coin shortage, none of the mint facilities in the US placed mintmarks on the coins they produced. This practice ended in 1967 and in 1968 mintmarks were again placed on US coins.
? Do you mean KEY DATES? A key refers to the date/mintmark combinations that are difficult to find (or expensive) in a series.
If the coin has a Mintmark it is on the back of the coin, above the letters DO in dollar.
Australian coins minted at the Melbourne Mint do not have a mintmark.
Yes, mintmark is one word.