The corbelled dome we are building is made of a continuous course of narrowing spiral of flagstones
You can see it better here in the outline.
We could have build it in a different way by creating separate rings of stone that lay on top of each other and get gradually smaller in diameter.
That would mean fitting the last stone of each circle in the space between the first and the second to last.
The spiral has the advantage of not having to do that kind of 'last stone' fitting on each row.
But is it as strong? That is the question.