The IMPERIAL series has precision-round Imperial Crystal nylon treble strings in medium tension with the famous bass strings of the Classic series and differ in tension (black = low, gold = medium, red = medium, blue = high);Imperial treble strings (the same for all models);Precision round Imperial Crystal treble strings in medium tension;E1 medium: .0287"/0.73mm, tension 8.4kg/18.52lbs;B/B2 medium: .0327"/0.83mm, tension 6.18kg/13.62lbs;G3 medium: .0406"/1.03mm, tension 6.16kg/13.59lbs;