# What do the two lines '||' in Kullback-Leibler Divergence mean?

I have just come across the Kullback-Leibler divergence in an article. This is the first time I am seeing it. What intrigued me are the two lines '||' in $D_{KL}(P‖Q)$. Below is the formulae.

I am not sure what they mean. Do they mean 'with respect to'?

Can anyone kindly clarfiy this for me?

• They don't mean anything really, it's just a way to separate $P$ and $Q$. Nov 24, 2017 at 4:43
• And to show it is non-commutative in $P,Q$ (in the way as $p(x|z)\$ ) Nov 24, 2017 at 5:01
• $D_{KL}(X\| Y) = \mathbb{E}_X[-\log \frac{p(X= x)}{p(Y=x)}]$. Nov 24, 2017 at 5:22