The Deeper Symbolism of Freemasonry from The Meaning of Masonry by Walter Leslie Wilmshurst.

“the three-sided emblem at the top added to the four-sided emblem beneath making seven, the perfect number; for, as it is written in an ancient Hebrew doctrine with which Masonry is closely allied, ‘God blessed and loved the number the seven more than all things under His throne,’ by which is meant that man, the seven-fold being, is the most cherished of all the Creator’s works.” [via]