What type of fishing line for setting downbearing?

[Brad Smith]

Hi List, 
Does anyone have the correct name for the type of fishing line best used for checking/setting downbearing? 
I saw it in a class, but can't remember the type of line that was used. 
It was clear/opaque, and when it touched the bridge, you could immediately see the contact 'through' the line. 
 
Best regards, 
Brad Smith, RPT 
www.smithpiano.com