Ron Guth: Beginning in 1925, the dates on the Standing Liberty Quarter Dollars were recessed, giving them more protection from wear.
Fully-struck examples are the most desirable and much attention is focused on Liberty's head. So-called "Full Head" examples (those with complete details) often bring considerable premiums over poorly struck examples, but attention should also be paid to the rivets on the shield. Any coin with a Full Head and full rivets is a true prize. Mintmarks on this type ("D" for Denver or "S" for San Francisco) appear on the obverse just to the left of, and slightly above, the date. The small M that appears to the right of the date is the designer's initial.
All date and mintmark combinations in this series are affordable, but Full Head versions of every date command a premium. Sometimes, the premium difference between a Flat Head and a Full head is substantial. For example, the 1927-S is a condition-rarity that becomes quite elusive and valuable as it approaches the Mint State level, and with a Full Head, the value skyrockets (see the PCGS Price Guide for similar examples).