Having two or three manuals makes it possible to have quick changes of sound during a piece. The player can also play on two manuals at once: one with the left hand and one with the right. This is particularly useful to make a tune louder than the accompaniment (on a piano this can be done by pressing harder). The manuals can also be coupled together, for example, pulling out the “Swell to Great” stop will make all the sounds from the Swell come out on the Great as well. On an organ with mechanical action the keys of the Swell will be seen “playing by themselves” like a pianola, but on some older organs it can be hard work for the organist’s fingers when the manuals are coupled as it makes the action very heavy.