You got a Raspberry Pi Model B+ that you want to have work with some old kit or plate or breakout or something and it uses the classic 26-pinout. Very frustrating, it is...unless you have this ingenious 'downgrade' GPIO cable. It has a 40-pin socket on one end, but only the first 26 pins have a cable and the other end matches any sort of Cobbler or PermaPi or what-have-you.
Cable is 6" long, and pin #1 is marked with a white wire. This cable is not compatible with the Raspberry Pi Model A or B - It's for the B+ only!