Board Thread:Lore Discussion/@comment-24374833-20140103222000/@comment-24358064-20140103223948

The Pocket Guide to the Empire, First Edition, mentions that historically the holds were governed by kings, clan chieftains or councils, depending on the individual hold. So even though nowadays they're all governed by Jarls, I imagine succession laws will still vary widely between the single holds. In some it will probably be simple succession to the firstborn child, in others there might be a hold-level moot (council) to determine the next Jarl. Also, all Jarls are vassals to the King of Skyrim, so I imagine he has some say in that as well (especially if there are no relatives left in a hereditary hold or something).