What are your favorite LaTeX macros?

I have quite a few collaborative writing projects going on in parallel now. One side-benefit of collaborating is that you learn neat LaTeX macros that your co-authors have developed that end up saving lots of time (or making the TeX equations more readable). Some people invent a whole set of macros for each paper (so that the macros stand for semantic concepts like “input variable”), but I do that mainly for small stuff, like different epsilons. What I do have are macros for are

  • font types and weights : using \mathrm{}, \mathcal{}, etc. is for the birds
  • functions : KL-divergence, mutual information, conditional mutual information etc. I get in trouble sometimes because I use the I(X \wedge Y) instead of I(X; Y) for mutual information, but we can change that in the macro!
  • norms, inner products : these are just functions anyway
  • epsilons : this helps keep different epsilons clearer, actually, and makes debugging proofs a little simpler.

I somehow never get around to making macros like \cX for \mathcal{X}, but maybe that would make my life easier. The nice thing about macros for functions is you can put in auto-sizing delimiters in the macro, saving some \left and \right‘s. What are your favorite things to make into macros?