Think, for example, of football referee signals, baseball signs, astrological signs. Any time you make up a secret code or set of signals you are making your own signifying system. There may be some kinds of signs that seem less arbitrary than others. Saussure discusses whether symbols, such as the use of scales for the idea of justice, are innate or arbitrary, and decides that these too are arbitrary, or based on community agreement. Think, for example, about the sounds attributed to animals.
Interjections also differ.
Admittedly, Saussure is not very interested in how communities agree on fixing or changing the relationships between signifiers and signifieds. The easiest example of this is a sentence, where the words come one at a time and in a line, one after the other, and because of that they are all connected to each other. One of the questions philosophers have puzzled over for centuries is whether ideas can exist at all without language. In other words, from Saussure's point of view, thought cannot exist without language. Sound is no more fixed than thought, though sounds can be distinguished from each other, and hence associated with ideas.