In the modern instructions, geometry is said to be the basis on which the super6trueture of Freemasonry is erected; and in the Old Constitutions of the Medieval Freemasons of England the most prominent place of all the sciences is given to geometry, which is made synonymous with Freemasonry. Thus, in the Regius Manuscript, which dates not later than the latter part of the fourteenth century. the Constitutions of Freemasonry are called “the Constitutions of the art of geometry according to Euclid,” the words geometry and Masonry being used indifferently throughout the document; and in.
In the Harlefan Manuscript, No. 2054, it is said, “thus the craft Geometry was governed there, and that worthy Master (Euclid) gave it the name of Geometry, and it is called Mosonne in this land long after.” In another part of the same manuscript, it is thus defined: “The fifth science is called Geometry, and it teaches a man to mete and measure of the earth and other things, which science is Masonry.”
The Egyptians were undoubtedly among the first who cultivated geometry as a science. “It was not less useful and necessary to them,” as Goguet observes (Origine des Lois, Origin of the Laws, I, iv, 4), “in the affairs of life, than agreeable to their speculatively philosophical genius.” From Egypt, which was the parent both of the sciences and the mysteries of the Pagan world, it passed over into other countries; and geometry and Operative Masonry have ever been found together, the latter carrying into execution those designs which were first traced according to the principles of the former.
Speculative Freemasonry is, in like manner, intimately connected with geometry. In deference to our operative ancestors, and, in fact, as a necessary result of our close connection with them, Speculative Freemasonry derives its most important symbols from this parent science. Hence it is not strange that Euclid, the most famous of geometricians, should be spoken of in all the Old Records as a founder of Freemasonry in Egypt, and that a special legend should have been invented in honor of his memory.
the source: Mackey’s Encyclopedia of Freemasonry