Common is the trade tongue — the mongrel dialect of ports, markets, garrisons and roads, assembled out of whatever the traffic of a region brought together and belonging properly to nobody. It is nobody’s first language and almost everybody’s second, which is precisely what makes it useful.
It behaves as any Language does: each Index band sets the complexity of what a character can express and follow, and the lesser of two speakers’ Indexes governs the exchange between them.
What distinguishes Common is the shape of its ceiling. It is broad and shallow. It has ample vocabulary for cargo, coin, distance, danger and law, and it will carry a negotiation or a warning across a frontier without trouble. It is markedly poorer at everything else — nuance, allusion, courtesy, verse, the precise word in a legal or theological argument — because none of that was ever what it was assembled for. A character can be entirely fluent in Common and still be quite unable to say anything graceful in it, and speaking it in a court where the vernacular is expected says something about the speaker whether they intend it or not.
Accent follows the ordinary rule of Eloquence Index × Language Index, though since Common has no native speakers, a heavy accent in it is unremarkable and betrays only where the speaker learned it.