The metal marker from the Forest Home Cemetery at Forest Park, Illinois, marks the grave of a Mason. It is the metal reproduction of what is perhaps the most recognizable emblem of the Freemasons, the square and two compasses. In this example the letter “G” appears in the middle of the emblem. Often the emblem is seen without the letter “G”.
Each component of the symbol represents a different Masonic orthodoxy, though, these are not hard and fast:
The compasses represent the boundaries of wisdom a person should have the strength to circumscribe and stay within.
The square symbolizes virtue in all actions, just as the expression “square deal” means treating people with fairness.
The letter “G” seems to have more than one meaning. It could possibly mean God, as in the creator of the universe; or Gimel, which is the word for the third letter of many Semitic languages. The number three is significant to many Masonic rituals and beliefs. Some also believe the “G” may represent geometry.
Gravely Speaking 
That Freemasonry has in her letter “G” and its connotations a relationship with this ancient association of “letters four and science five -“ that is, of Deity and science or knowledge – is not remarkable – rather it would extraordinary if she had not. In all ages and all religions, man has interwoven together his thought of spirit and matter, his ideas of relative and absolute. Freemasonry’s “G” is but another of these conceptions, expressed in a symbol. If the symbol now used – a Roman “G” – is less fitting for an art concerned especially with squares than was the original Gamma, it at least should receive the reverence due a respectable age. Even those whose ideas of the fitness of things would be better satisfied if our “G” were Gamma, would hardly subscribe to an effort to change now. Mackey, the great Masonic authority, regretted that the Roman “g” ever found its way into our symbolism, and read the “G” as a substitute for the Hebrew Yod, which in turn is a symbol of the tetragrammaton, or four-letter word. Unquestionably the “Lost Word,” the very heart of the Masonic system, is represented by the Yod, but it is a far cry to include also Geometry in that representation. The Greek Gamma, (of which our roman “G” is a substitute) however, did represent both the ineffable Name and the greatest of the sciences. Three Greek letters which spell our name for Deity can be monogrammed to make a modern Roman “G” inside a square and compasses. However corrupt the geometrical form of the Roman “G”, and however much more illuminating it might have been had we continued to use the Greek Gamma of Pythagoras, what we have adopted and made so integral a part of our Masonry that it is in every English speaking Lodge in the world, is far to sacred and familiar ever to change.