Something personal

Everyone has keys. You can't escape it. Sometimes you forget them and you have to hope that someone else can let you in. A set of keys can tell an enormous amount about a person and who they are. For example, blood donors at Sanquin receive a keychain with their blood group printed on it that can be hung on a set of keys. (Fig 1) This makes it immediately clear that the person donates blood and what their blood type is. You can also choose not to attach any extra frills, perhaps because you like to keep it simple and organized. But what if we zoom out: what can something as simple as a set of keys tell us about society?