How to make a double seam in sheet metal work?

[janwide]

Does anyone know what a "dubbelfals" looks like and how to perform it in sheet metal work? I assume there is also "enkelfals." When would you use it, in that case?

 

[MaZtoR]

Above a slope of 14 degrees, only single seam is needed. Below that, double seam is required.

http://www.teokonsult.se/plat/plfackor.htm

There are pictures of the different seams in profile.

 

[janwide]

Thank you!