Dropped D tuner. Instantly go from standard to "dropped d" tuning. Works best with a stabilized non floating trem, but I have used it with a floating trem with acceptable results. No mods necessary. This one was never used...I bought 2 and 1 of my Floyd Rose trems was recessed into the body of my guitar (like Steve Vai;s) so I couldn't use it. May not work on some cheaper "licensed" Floyd Rose tremolo units.