I have the Ligne chart for crystals and lets say
20 14/16 is 47.1mm so how come when I measure the actual crystal I do not get the same readind of 47.1 mm on my calipers?

What measurement do you get?

Here is what I read about it:

If your crystal measures 47.1mm you divide by 2.256 to get 20.87 Lignes.

Remove the 20 Lignes before the decimal place and multiply the 0.87 by 16 to get 13.92 or 14.

So the crystal should be 20 14/16 Lignes.

ok 47.1 is 1.854 of and inch
but my crystal measure 1.859 of an inch which= to 47.2 mm and the sticker shows 20 14/16

It has been my experience from my crystal cabinet that the actual size range of one size set of crystals eats up at least 1/2 the scale to the next larger and smaller crystals.

Thanks Dave

Group elders, feel free to correct me if I am off base here... but,

If I measure across the bezel and get 20 14/16... it makes sense the crystal is .1mm larger to give the interference fit for the crystal to capture in the lip of the bezel...

If it was exactly the same size, it would not stay in place without glue.

That makes sense to me as a good place to start. A sheet of printer paper is about .1 mm thick.