These words refer to something that is possessed or can be used by two or more people or in two or more situations. Mutual and reciprocal are close in meaning, suggesting most simply some interaction between two parties, in which the same thing is given and taken on both sides: their mutual respect for each other; a reciprocal lowering of tariffs between the two countries. Reciprocal is the more common term in official or technical contexts. Strictly speaking, neither would be used for groups of three or more unless the indicated relationships were true for every possible combination of two. Mutual , is commonly used for any similarity of interests, views, feelings, even when the relationships are not reciprocal : a mutual interest in hiking that drew the two boys together; a mutual excitement felt by the whole audience.

Common escapes the objections that may be levelled at mutual , referring widely to something possessed by two or more people: speaking a common language; a common interest in silent films. Because common can also mean ordinary, or even cheap or debased, the word’s ambiguity sometimes prohibits its use, a fact which may justify the substitution of mutual . A mutual friend would unmistakably be one that two or more people were friendly with, but a common friend might be a vulgar or coarse person.

Joint exclusively emphasizes the actual possession of something by no more and no less than two people: a joint bank account. Since it suggests a unity, as of two halves within a whole, the word would not point to mere similarity of interests: a joint business venture in which both put up an equal amount of capital. Interchangeable involves a different concept altogether, applying to parts or elements that can be moved from one setting to another without loss of function or can be used one in place of the other: standardized shaving heads that are interchangeable from one model of electric razor to another.

• Jackass and kookaburra are interchangeable words for a well-known Australian bird.

Shared gives a tone of even greater emotional warmth than is true for mutual ; it also can indicate something actually divided up and used by two or more people: our shared lunch of bread and cheese. More generally, it can replace both common and mutual in many contexts, often with a gained clarity or sense of intimacy: our shared language; pointing out that shared interests often lead to marriage. In some cases the word would be unsuitable: a shared friend. Here, an odd tone of possession blurs the intended meaning.