Necessary condition for grammar ambiguity

Question

In my notebook I wrote:

The necessary condition for grammar ambiguity is

1. It contains the rule A->BB, where A and B are non-terminals.
2. OR it contains the rule A->a|b, where A is a non-terminal and {a,b} are terminals.

Would you please confirm or refute this statement?

Answer 1

That's not true because there are other ambiguous grammars that don't have either of those rules.

For example cc can be produced by A -> Bc -> cc but also by A -> cC -> cc in the following grammar:

A -> Bc | cC
B -> c
C -> c