The Common Tongue was brought to Westeros by the Andals and the Rhoynar. It is spoken in the entirety of Westeros. There are regional accents, as "Pyp had traveled the Seven Kingdoms with a mummers' troupe, and bragged that he could tell what you were and where you'd been born just from the sound of your voice", but there is no lack of comprehensibility, in any region. Some of regional accents can be geographically limited to a single city, as King's Landing.
The largest dichotomy of speech appears to be between the lowborn and the educated, most likely due to a long history of prescriptivist tutors for the highborn young. (For example, Lady Mariya corrects Lady Amerei's use of the word hung to hanged, in regards to Merrett Frey's death.) There are differences that are not exhibited in the orthography, as it is possible to distinguish highborn from low accent merely by listening: "Doran!" called some highborn voice. "To the spears!" These differences encapsulate both highborn vs. lowborn as well as regional dialects:
- "How could you tell I was of noble birth?" [Sam asked].
- "The same way you can tell that I'm half Dornish." The statement was delivered with a smile, in a soft Dornish drawl.
Mycah the butcher's son's few lines shows the lowborn patois:
- "She ast me to, m'lord," Mycah said. "She ast me to."..."It's not no sword, it's only a stick." 
Another stablehand in the Red Keep has other non-standard usages: "She don't know me," he said, "but I knows her, oh, yes...I says, come." And one of Ser Gregor's men uses the colloquial past tense of eat, et.
Even the warriors of the Mountains of the Moon, who have not had much contact with anyone for at least several hundred years, have little difference in their speech.
- Shagga glowered, a fearsome sight to see. "Shagga son of Dolf likes this not. Shagga will go with the boyman, and if the boyman lies, Shagga will chop off his manhood."
References and Notes
- A Game of Thrones,p.259
- A Feast for Crows,p.556
- A Feast for Crows, p.450
- A Feast for Crows,p.41
- A Feast for Crows, p.678
- A Game of Thrones, p.150
- A Game of Thrones, p.538
- A Feast for Crows, p.403
- A Game of Thrones, p.607
- Languages Corpus at DothrakiWiki
- A Game of Thrones, Chapter 72, Daenerys X, p 800.