Using itemprop without itemtype

Question

If I understand it correctly, it is possible to use itemprop without itemtype. Is this true?

If so, what's the difference in terms of SEO? Is it better to use it with itemtype? I am guessing yes, obviously, but what if the Microdata is not equal to the site you have? For example, I have a dictionary and I could use book, but would this be a good thing to do? I don't know how Google will see this. It would be not fine to get a penalty because of bad use.

So: Do I have to use schema.org schemes?

Answer 1

It's valid to use itemprop without itemtype, but any meaning of the itemprops will be private. In order for that meaning to be shared with public microdata consumers, it will be necessary to associate it with an itemtype that is understood by that consumer.