Common is the commonly spoken and written language used over much of the known worlds by descendants of the Eladamri who would have originally spoken Standard. Common has many dialects and variations, but the basic sentence structure and syntax has remained relatively similar. The main variations are in vocabulary, though some isolated cultures have been known to develop very different languages.
The universal alphabet for Common is called Egregts, which contains 26 characters. It is one of about a dozen different recognized variations, which share a common character set but sometimes as many as 34 letters. Egregts is important because the 26 characters share roughly the same interactions and make roughly the same sounds across all known dialects of Common. The character set was in use by at least the midpoint of the Mage Wars, with scattered traces dating back even further. It is believed to be the same alphabet as used by Standard, but with some simplification due to the loss of Scholar.
The Pendragon's Common
Common has many sub-dialects and regional variations. Words can frequently have slight or large differences in meaning, which also changes over time. The first person to really travel widely throughout the known worlds(and to be widely respected), was Eieber, first Pendragon of Slayer Dragons. His lieutenants and subordinates, many of whom were his compatriates from Ataya or had spent many years at his side, often stayed behind on other worlds for long stretches.
Thus, the Atayan dialect of Common became widely known as "The Pendragon's Common", a tradition which continued throughout the Ages of the Alliance. In fact, this persisted whether or not the actual Pendragon was Atayan or used a similar dialect, and became something of a running joke throughout the known worlds. Any time there was a disagreement about the meaning of a word, the first person to claim their definition was "The Pendragon's Common" usually won.