1873 France: Variety 'Fond Ligné'

I have an 1873 25c blue, which purports (on expert authority) to be a 'fond ligné' variety. I do not think that the variety does much for its CV, but I just cannot work out 'fond ligné'. (A Frenchman attempted to explain, but lost me!)
Can anyone help me out, with a simple explanation? What should I be looking for?
Thank you.

Fond ligné appears to mean quadrille paper.
This might help:
https://books.google.com/books?id=E...tely&f=false

Assuming that this is from the group Yvert 60A/B/C, the catalogue doesn't use the phrase "fond ligne". It includes illustrations of the frame variations at the top right. Share a bottle of Chablis with your friend and ask him again!

"fond ligné" means lined background. There is lines in the background on this stamp. They are wavy lines. Could be what that make the variety. Daniel

Looking in my Maury -Timbres de France- your stamp to me (not an expert) looks like a 60II, which in 2011 had a CV of between 30 and 40 Euros depending on the shade of blue. That is apporx 15-20x the CV of the other versions of the stamp. If it was unused, the CV would have been in the 1650-3000 Euro Range. But as used, its a nice stamp but not worth splurging on the Dom Perignon either.



What makes it a type II is that little blob of ink to the left of the top right cross.

According to this seller, 'fond ligné' means background of head made up of horizontal lines. This type of variety is usually found when a solid background is underinked and you find that what was solid was actually lines spaced close enough together for the ink to merge when printed.

Sincere thanks to all!
I know that the stamp is Type II (SG199a; Yvert 60B). And that Yvert, at least, does not list the variety for this value. However, it is signed Calves and when I checked with the expert he confirmed that he had expertised it as fond ligné. But that was as far as my French could go with his explanation! I'll try the bottle of Chablis GeoffHa but, failing success with that, I'll check out the other very helpful suggestions. I've found the Washington Stamp Company reference, billsey. They obviously have a clear view on the matter, so I'll work on making sense of that.
Thanks again folks.