Geometry 

Geometry is one of the Seven Liberal Arts.  It teaches us about Mensuration, Weights and Measures, and other handicrafts.

It is said that Geometry was once considered the same a Masonry.  A working Knowledge of Geometry is needed to construct buildings.  The "G" in the center of the Square and Compasses is said by many to indicate Geometry.  It may also have other meanings.

The secrets of Geometry were at one time known to only a few.  These secrets were heavily guarded by those who knew them.  The secrets of Geometry allowed the Masons to construct buildings that others could not construct. 

Many of these secrets about Geometry are now taught to everyone in our schools.  They are now known to all who take the time to learn them.

Thus a working Knowledge of Geometry seems to have been one of the closely guarded Secrets of Freemasonry, at least in the past when most people were ignorant of the topic.  Actually, based upon my years of teaching math and physics, most people are still ignorant of Geometry.