In 1866, mint engravers added a motto bearing the words IN GOD WE TRUST to the reverse of the Quarter Dollar. This motto appeared on U.S. Two Cent pieces first in 1864, then slowly made it to larger-sized silver and gold coins by 1866.
The mintage of the 1866 Quarter Dollar was small because most of the Philadelphia mint's attention was focused on minor coins, especially the new "Nickels." In fact, the 1866 With Motto Quarter Dollar boasts the smallest mintage of any other date since 1827. As a result, this the 1866 With Motto Quarter is scarce in all grades, but sufficient examples have survived to keep the price of Mint State coins at a reasonable level compared to other rarities in the series.
Using the Gem example illustrated above as a reference point, it appears that some examples come weakly struck on the center of the reverse, particularly in the area of the top left portion of the shield and the surrounding feathers.