RTH6400 Honeywell - where does the blue wire go.

it all depends how things are wired inside your furnace downstairs on a basic system blue is usually a common - C and yellow is for cool - Y just leave the jumper in from R & RC R is 24v power and RC is 24v power in Cool yuo will just have to squeeze the new wire in with it before you get to far i would open up the doors on you furnace and see where the connections are in there so you dont blow any fuses