A series of markings on the underside of a pillar fragment appear to show carefully drawn and scribed lines showing the plan of a wall with a three-lobed attached column.
There is also a set of four circles, neatly created using a compass, which could be the plans for the bases of small columns which still exist on the upper surface of the base.
The rare find was made by Dr Mary Márkus, a specialist in Medieval architectural stone, who is employed by Historic Scotland to catalogue and record the carved stone collection at the cathedral.
She said: "What is so thrilling about this is that it gives us a direct insight into the mind of an early 13th-century stonemason.
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"It is possible to see the working out and the thinking as the mason effectively made notes on what he intended to do, checking that it would work."
Dr Márkus compared the measurements of the four-lobed markings with the small column bases on the upper surface of the stone, and found they were a close match.
"It looks as if the mason was making a working drawing for another mason, to show exactly what was to be carved on the upper surface.
"So we have a surviving example of both the plans and the end result," she said.
Only two other examples of mason's plans are known in Scotland, one at Dunfermline Abbey and some later, 15th-century, markings in Rosslyn Chapel.
Hugh Morrison, Historic Scotland collections registrar, said: "It is quite remarkable to have found what appear to be mason’s plans for part of one of Scotland's great historic buildings.
"They come from an era before professional architects when masons were largely responsible for design and building.
"They were people of remarkable skill and ability, but there are few surviving written records to tell us about who they were and how they went about their work.
"So to have this direct view into the life and work of one of these men, from so many centuries ago, is quite wonderful."
The cataloguing and recording of the stones is part of an ongoing project by the Historic Scotland collections unit.
(GK/JM)
