Kyle was the central of the three districts in the sheriffdom of Ayr (Ayrshire), which was divided naturally by its three primary rivers all running in a generally westward direction to flow into the firth of Clyde. The River Irvine forms the northern boundary of Kyle with Cunninghame; the River Doon established its southern boundary with Carrick. Additionally, Kyle itself was sub-divided into two parts. To the north of the River Ayr was "Kyle Stewart" (sometimes called "Stewart Kyle" or "Walter's Kyle"), lands held by the FitzAlans (the future Stewart Kings of Scotland) since the 12th century. To the south was "Kyle Regis" or "King's Kyle", lands historically retained by the monarch under royal authority from the royal castle at Ayr.