a5191371_ing_the_Right_Strike_Indicator__2
[0] Fly Line [1] Quiet presentation for spooked fish; does not effect presentation or casting [3] Any depth; calm water [2] 
Can be hard to find in stores; indicator doesn't always stay above water so in fast water or low light conditions, can be hard to follow and you may miss strikes.
Picking the Right Strike Indicator
2