Tried to work this out but can't.

I understand that hooks are sized 0 to whatever and that a size1 hook is BIG
and a size 10 hook is teeny, but whats the alternative method wherein a
slah and a zero follows? e.g. 4/0.

Whats the trailing zero mean and why is it always a zero? I assume it isnt
ALWAYS a zero, but I have never seen anything other than a zero.

So, why two systems (1,2,3 etc and 1/0, 2/0, 3/0) and what do they mean?

David