I ask the question because if you go and have a look at a 3DL sail you will spend a long time looking for a seam of any type.... there are none <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Now, As I am an unemployed bum this week (wayne would contend that I am unemployable..) I have time to dig up plenty of info. From the 2007 Finn class rules:-
.pdf]http://www.sailing.org/tools/documents/FIN2007_CR_240307-[469].pdf

Rule G.1.7 (b) continuous layers or fibres crossing the seams are prohibited.

So if a 3DL sail had seams it would be class legal in the finn without the current process of cutting the sail and regluing it to make the seams.