Pennsylvania's state symbol is the KEYSTONE, which is represented by three letters in a crossword puzzle. The KEYSTONE is a wedge-shaped stone that is the central piece of an arch and holds all the other stones in place. It is also the symbol of the Commonwealth of Pennsylvania, which was adopted in 1802. The word 'KEYSTONE' is derived from the Latin word for 'doorstone, ' which is fitting since it is the stone that holds the door together.