What is a 'w' mint mark?
It means the coin was struck at the West Point mint (commemorative coins only)
Keep in mind though that there are two coins that may appear to the inexperienced coin collector to be struck at the West Port Mint. Both the Mercury Dime (1916-1945) and the Walking Liberty Half Dollar (1916-1947) have Adolph Alexander Weinman's initials on them which may look like a "W" mintmark. However, none of these coins were ever minted at the West Point Mint and are not mintmarks.