Brian, the burl veneer is sliced at about 1/42" so when you buy eight sequenced leaves they are all taken from a piece of solid burl under 1/4" thick. That means that number one and number eight are very close to identical and the bottom of any leaf is exactly like the top of the next. In a four way match two of the seams use these perfect matches and the third and fourth are usually so close as to be taken for perfect.
The mirrors themselves are used simulate what different matches will look like without cutting the veneer to see them.
I hope this helps. It’s harder to explain than it is to do. :-)

Ian, thanks, I find that they have not only one of the best selections but the best organized and photographed site and purchasing setup.

The early bird gets the worm but its the second mouse that gets the cheese.