On either side of the manuals there are rows of "stops" which look like knobs. The stops can be pulled out or pushed in. When a stop is pulled out, it turns on some sets of pipes. The organist can choose whether to play loud pipes or soft pipes, flute-sounding pipes or brassy pipes, sweet pipes or harsh-sounding pipes. As the organist plays, he/she does not just have to think about the right notes. He/she also has to think about the sort of "voice" that the organ should play in. He/she can play different ranks of pipes together by pulling out several stops. Some pipes, usually the biggest decorated pipes at the very front of the organ, are used only for the grandest music. By tradition, these pipes are the symbol of the "Voice of God".