When you buy a pdalier Zoom without pedals (including 505II), it's nice to have a ct. It is used to control the wha-wha, volume, the gain of the distortion, the speed of the chorus or flanger, and pitch ,.... IN SHORT, IT IS ESSENTIAL!! I've had little time, I comand in a music store, and 1 week later I had. I got 49 (price list). It's a bit expensive for expression pedals, but worth it. It is not big and very easy to install (no need to jack more!). His only dfault the price (which is I put a 9 / 10).