The Origins of the T-H Sound

Early on there was a regional divide in writing the T-H sound. In the north they used the Irish derived F, and in the south they used the runic symbol Thorn. But by the 8th century these two different ways had spread throughout the island. Both letters were being used interchangeably throughout the north and the south. The Normans didn't like any letters that weren't Roman, so they got rid of both F and Thorn. And that was the point at which all of those English words finally got their modern T-H spellings.