Summary: This is the volume dial on a home HiFi speaker. It is an example where there is a one-to-one mapping between logical and physical states, but the dial has to be decorated to make this a visually exposed state.
This speaker volume control dial is another example of a device with a one-to-one mapping between its setting and the thing that it controls (volume). It is drawn in the physigram here as an 11 state device, but is, in fact, continuous, and is almost certainly an electronic control via a potentiometer, rather than digital (we didn’t disassemble the speaker to find out).
If the knob did not have any decoration, although you might be able to feel where your hand is when turning the dial, you could not see what state that represents, because it is perfectly round. It may sound quiet enough now, but what is it going to be like when trumpets start? That is, while it is one-to-one in terms of its physical movement, it does not have exposed visible state because of the circular symmetry.
That is why dials of this sort almost always have some sort of decoration. In this case there is a slight indentation on the dial, and painted dots on the speaker.
