HG stands for Hyper Glide, IG for InterGlide. Shimano first came out with HG chains in '91. In '94, Shimano went from 7 speed to 8 speed and in '95 they introduced the IG drivetrain. ... The main difference between IG and HG is the chain width. IG chain is 7.1 mm wide, HG 6.6 mm wide.