The Square and Compasses is one of the most prominent symbols of Freemasonry. The "G" in the middle represents God, otherwise known inside Lodges as "The Grand Geometrician of the Universe", since Freemasonry demands belief in no specific faith, simply a belief in a "Supreme Being". It has also been ascribed to the Worshipful Master's word.
Some also claim that it represents Gnosticism or Saint Germain.
The symbol of the Square and Compasses is a trademark and has been registered in some jurisdictions.  [1]

This image was created for the Grand Lodge of British Columbia and Yukon in 1993.  [2]
Own work (Original text: own work)
MesserWoland