Common is, as the name suggests, the most common language used by humanoids on the Prime Material plane in order to communicate with each other.
While many species have their own racial language, and learn Common as a secondary language, humans tend to learn Common as their primary language. As such, it's a language sometimes associated with humans rather than as a language for mutual trade and communication.
Common is said to have originated in Sigil, which may conflict with the claim that Planar Common developed from the Common spoken by visitors to Sigil from the Prime.[1] Common and Planar Common are similar enough that speakers can mostly understand each other.
Common also has an associated sign language for those who cannot hear, or those who wish to otherwise communicate non-verbally.
Usage
In 125 YFHR, a Riddling Skeleton in the Dead Nations posed a riddle to another skeleton: "Think of words which end in '-GRY.' Angry and hungry are two of them. There are but three words in the Common Tongue... what is the third word? The word is something that one uses every day. If thou hast listened carefully, I have already told thee what it is."
The riddle's answer was "tongue" as there are three words in "The Common Tongue."
Background Information
- "Common" is meant to be the fantasy equivalent of whatever language the players speak: e.g. for English-speaking players, Common would be English.