> I am not sure if I understand what you mean. But let me put it in a way 
> that I can understand. There are two special stones. One is at the end of 
> one arc and at the beginning of the next one. The other is the one in the 
> middle of an arc. Referring to my picture, the latter is the highest one 
> in the arc and has a horizontal lower line. The other is 45 degrees 
> turned. No matter how you position the other stones. The corner one 
> advances sqrt(2) times the side of that stone every arc. The middle one 
> can only advance by the side of that stone. So you'll end up with 41% 
> space between the center stones of two consecutive arcs . That does not 
> happen in real life. Either because not all stones have the same size or 
> they are cheating in another way.

Well they are forced to 'cheat' a little, but with skill it doesn't look too 
obvious.

In a perfect pattern the ones at the end of the arc would be pretty close to 
square and smaller than the ones in the middle of the arc (see the POV-Ray 
image I just posted). In a perfect pattern these differences could be fairly 
apparent, but the craftsman randomizes the pattern a little using different 
block sizes and different mortar gaps, so the 'cheating' becomes less 
apparent.

As shown in the picture posted by Thomas, they don't even always pick square 
blocks at the outer extremity of the arcs, as very apparent straight lines 
would then form. As shown in your picture, if they used identical square 
blocks they would indeed end up with large mortar gaps in the middle which I 
think would be dissapointing for all concerned :-)

Regards,
Chris B.

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