What's the correct type of brackets to use in a matrix?

Question

I had my first day of Linear Algebra today, and we got introduced to matrices, one thing that kept nagging me though, was that I kept seeing matrices written in a few different styles, namely

$$ A = \left[ \begin{array}{ccc} a & b & c \\ d & e & f \\ g & h & i \end{array} \right] $$

as well as

$$ B = \left( \begin{array}{ccc} a & b & c \\ d & e & f \\ g & h & i \end{array} \right) $$

and on really rare occasions I've even seen

$$ C = \left\{ \begin{array}{ccc} a & b & c \\ d & e & f \\ g & h & i \end{array} \right\} $$

So my question is, which of these is considered most correct to use? $A$, $B$, or $C$?

Answer 1

They are all used, mostly interchangeably, depending on the style of the author.

Answer 2

I prefer $A$. Some author $B$. I've never seen $C$.