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

“The inward development which the second degree symbolizes is typified by the lowering of the triangular flap of the apron upon the rectangular portion below. This is equivalent to the rite of Confirmation in the Christian Churches. It denotes ‘the progress we have made in the science,’ or in other words it indicates that the higher nature of the man, symbolized by the trinity of spirit, has descended into and is now permeating his lower nature.” [via]