# Mutual Information Notation

I am confused about the difference which the use of ";" and "," causes in the following expressions for defining mutual information between X and Y

$I(X ; Y)$ and $I(X , Y)$

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There is no difference. The mutual information is defined between two random variables. –  Euclean Nov 12 '12 at 8:26
I believe it is because $I(X;Y) = I(Y;X)$. Mutual information does not measure a directional information flow, but how much $X$ can tell you about $Y$, and vice versa.