Used by the Swiss and the French a ligne is one twelfth of a Paris inch.
See the chart below:

here is the link to easily convert to mm from ligne
http://www.convertunits.com/from/ligne+[France]/to/mm

Peter, Thanks.
I was trying to inform the members as to the definition of a Ligne & you Greatly added to it. That is a great site to put in your favorites folder.

Actually I thought it had something to do with some King;'s big toe@