Using braces to label parts of an equation

Question

Given an equation, say Bayes' rule,

$P(X \mid O) \propto P(X)P(O \mid X)$

how do I label the different parts of the equation using curly braces?

Answer 1

I think you're after the \underbrace and \overbrace commands, which you could use as

$\underbrace{P(X \mid O)}_{p_1} \propto \overbrace{P(X)P(O \mid X)}^{p_2}$.

Which results in: