IR kitty string is a 1.5 sized nylon string originally developed for softer silicone pads ( like the now defunct IR pad, hence the name), but still loved by many pros across the world for its interesting properties. The nylon material gives it a stiffer profile and makes it more durable (as other strings would wear too quickly on softer pads) but the middle thickness between the thin and fat kitty means that it doesn't feel too heavy.