Why is the skirret not depicted on the membership certificate?

A skirret is an instrument usually made of wood, shaped like the letter T, which acts on a centre pin, from which a centre line can be drawn, chalked and struck, to mark out the ground for the foundation of the intended structure. It is one of working tools used symbolically in masonic ceremonies but is not included with the other tools depicted on the membership certificate issued by the United Grand Lodge of England. e explanation of the omission of the skirret (nor is, the 24 inch gauge). Apart from the three pillars, the two globes and the “Furniture” of the lodge the certificate does not however show the tools as working tools but as the Moveable and Immovable Jewels of the Lodge. Thus the plumb-rule, level and square on the certificate are not the Working Tools of the Second Degree but the three Moveable Jewels of the Lodge. As such they are associated in the design of the certificate with the three principal officers of the lodge, who are themselves distinguished by those Jewels. The three Immovable Jewels (the rough ashlar, perfect ashlar and tracing board) are likewise pictorially related to the same pillars.