Joined: Apr 2007
Posts: 821
Likes: 158
From: Ottawa, Canada
Bikes: Raleigh Classic 15, 84; Miyata 912, 85; Miyata Ridge Runner SE, 85; Miyata 610, 86; Miyata 100M, 86; Miyata Valley Runner, 88; Miyata Triple Cross, 89; GT Karakoram, 90; Miyata Elevation 300, 91; Marinoni Touring, 95; Long Haul Trucker, 2013
Derailleur cables are noticeably thinner than brake cables, and have the two types have very different ends. You can't interchange them.
There are also different kinds of brake cables, and some different kinds of shifter cables.
What bike do you have, what brakes are on it, and what shifters?