Why 2-point correlation function diagram have only two external legs?

[ndung200790]

Please teach me this:
Why 2-point correlation function diagrams have only two external lines.Because of Wick theorem they would have many external legs.
Another thing is whether are there odd number point correlation functions(meaning they are different from 0).
Thank you very much in advanced.

 

[ndung200790]

Now,I have understood that,because of normal operator(N: operator) being in Wick theorem statement.

 

[dextercioby]

Well, because of the number of external 'legs' (2), they are called '2-point correlation functions', or '2-point Green functions'. If they had N-external 'legs', they would be called 'N-point correlation functions/Green functions'.