What does it mean when a penny doesn't have a letter under the date?

Question

I HAVE 3 PENNIES THAT DON'T HAVE LETTERS UNDER THE DATE, wHAT DOES THAT MEAN? I have a 1990,1967, and a 1976

Answer 1

'P'hiladelphia is considered the 'mother mint,' being the nation's oldest. By tradition, most 'P'hiladelphia coins don't have a 'P' mint mark, unlike 'D'enver.

'S'an Francisco and 'W'est Point are mints where specialty coins are minted, usually gold but sometimes silver or platinum.

There were other mints which are no longer in operation, such as New 'O'rleans, 'C'arson 'C'ity and 'D'ahlonega. These coins are much rarer and therefore usually more expensive due to their age, generally low mintage and scarcity.

Answer 2

The letter tells you what state the coin was struck.  No "mint mark" just means it was minted in Philadelphia