# Type-0 grammar and terminal symbols

The question is pretty short, but I've been thinking about it quite some time: Are terminal symbols that are not in the defined alphabet still valid?

• Not a real answer here, so I write a comment. If you define a grammar $G = (N, \{a, b\}, P, S)$ and have a production rule like $\alpha \to \beta c \gamma$, one may say that this rule cannot be in $P$. But all in all the question is not really profound. Why should anyone define a grammar with such inconsistency? – ttnick Jun 8 '18 at 14:06
• @PHPNick because it's not mine :) – just_deko Jun 8 '18 at 15:06