Interworld Common languages are languages designed to allow communication between the worlds involved in the worldgate events. There are two Interworld Common languages, or versions of the language, on which more below.
The common languages are an idea of convenience for authors and their worlds. Any sudden mix of isolated cultures requires and soon develops some method of communication - whether a pidgin mix ([definition]), everyone adopting the language of the most powerful nation or, in this case, a deliberately constructed new language.
The Interworld Common languages aren't perfect, but they are an answer to the question "what language are they all speaking?".
The primary concern with both versions of Interworld Common was to make a language that was both neutral and very easy to learn.
Interworld Common languages are shared property among Pro members (as far as a language we never see counts as a thing). They are also freely taught among interworld visitors. Anyone whose characters or worlds are involved in the worldgate events (or even just play at the inn, etc) can choose to say their people know the first and/or second Interworld Common without too much stressing about the details.
See also: list of languages